SPWLA-INDIA 3rd Annual Logging Symposium, Mumbai, India Nov 25-26, 2011

Transcription

1 DIAGNOSING COMPLEX FLOW CHARACTERISTICS OF HIGHLY DEVIATED AND HORIZONTAL WELLS USING ADVANCED PRODUCTION LOGGING TOOL-FSI: CASE STUDIES FROM MUMBAI OFFSHORE, INDIA Sunil Chaudhary*, M. S. Murty*, R. K. Pandey*, U. C. Bhatt*, Vibhor Verma, Ravi Sinha, Arun Pandey, Ajit Kumar, Konark Ogra *Oil and Natural Gas Corporation Limited, Schlumberger Copyright 2011, held jointly by the Society of Petrophysicists and Well Log Analysts (SPWLA) and the submitting authors. This paper was prepared for presentation at the SPWLA-INDIA 3 rd Annual Logging Symposium on th November 2011 in Mumbai ABSTRACT Production logging is used to diagnose well production problems by evaluating the inflow rates, entries of unwanted fluids and downhole flow regimes. Production logging results supply information for reservoir modeling, provide data to optimize the productivity of existing wells and plan drilling and completion strategies for future wells. These activities and needs has not changed with time, however logging environment has become much more challenging. For field development purpose in both primary recovery and enhanced oil recovery (EOR) phase, drilling highly deviated and horizontal wells is now a worldwide practice. It has made production logging not an easy task any more. In multi-phase environment, flow regimes and characteristics in highly deviated and horizontal wells are very different and complicated from those of vertical wells. Various factors like phase segregation, misting, recirculation of heavier phase and cross flow influence and complicate flow regimes. Conventional production logging tools based on center-point are found to be inadequate for providing accurate flow profiles in such complicated scenarios. In a highly deviated well, Gradiomanometer losses its accuracy and in horizontal section it doesn t work at all. A change in deviation as small as one degree causes large changes in fluid holdup and velocity. Such effects can confuse or even completely invalidate the fullbore single spinner for flow rates. Temperature sensor doesn t respond very well to fluid entries as they occur over a much larger interval than those usually found in vertical and near vertical wells. The downhole flow regimes in horizontal wells can be one or a combination various flow regimes. This makes the flow models developed for vertical wells of no use. Consequently it becomes difficult or almost impossible to achieve production logging objectives in horizontal or highly deviated wells by using traditional sensors and interpretation techniques. In order to address these challenges and to provide better answers, a new generation of production logging tools is specifically developed for highly deviated and horizontal wells. FloScan Imager (FSI*) is the state-of-the-art production logging tool serving to the same cause. FSI* is capable of evaluating phase wise flow rates and identify complex flow regimes like stratified flow, annular flow, plug flow, slug flow, recirculation, cross-flow and many with the help of its increased number of velocity and phase sensors. Three arrays of sensors: 5 mini-spinners, 6 E- probes and 6 O-probes spread all across the wellbore. It provides velocity and holdup profile along the vertical diameter of wellbore. Direct measurement of phase velocity and holdup profile enables accurate calculation of flowrate of different phases and reduces uncertainties involved with flow models. This paper discusses application of FloScan Imager (FSI) to resolve all kind of flow complications seen in highly deviated and horizontal wells in Mumbai high offshore. A series of case studies will be presented to showcase unique applications of FSI, the state-of-the-art production logging tool. 1

2 INTRODUCTION The Mumbai High Field is about 160 km North West off the Mumbai City and is the biggest oilfield in western offshore India, with an aerial extent of 1200 sq km. (Figure-1 and Figure-2) The Mumbai High Field is a multi-layered limestone reservoir with large variations in fluid flow properties both vertically and laterally, produces around 250k BOPD (~40% of India's oil production). In terms of reservoir heterogeneity The Mumbai high field is considered to be the one of the most complex fields worldwide. The field started producing in 1976 and a Pressure maintenance scheme using water injection was started in 1984, since there was unexpected and sharp pressure drop with initial production. The field is currently in decline phase with problems like increase in water cut and high GOR which are affecting the oil production the average field produces about 250,000 barrels of oil per day. Figure 1: Mumbai High Field Figure 2: Western India fields, with present status of Oil, Gas discovery Several oil and gas zones have been identified in this field, LIII being the most prolific with 90% hydrocarbon accumulations. The reservoir is essentially composed of limestone layers separated by the thin shale sections. The shale section divides the L-III reservoir into no of zones which are termed as A1, A2, B, C, D and E. Several thin shale sections further sub-divide A2 into seven sub layers. In the following section is discussed several challenges and problems seen while logging with convention production logging tools in this multilayered reservoir. CONVENTIONAL PRODUCTION LOGGING METHODS AND CHALLENGES A conventional production logging tool string usually includes spinners, electrical and optical probes for hold-up measurement, density, temperature and pressure sensors. The spinner is used to measure the total velocity of fluid mixture and density and hold-up probes are used to evaluate instantaneous hold-up of water, oil and gas. In order to correctly measure the flow profile, spinner velocities are converted to total flow rate and individual velocities for each phase are determined. Well fluid flow behaviour in vertical and near vertical wells is thoroughly understood over the period of time and results are derived accurately in this kind of environment. Highly deviated and horizontal wells present much challenging and complex logging environment for conventional production logging tools. With the advancement of completion technologies catering to the needs for primary as well as enhanced oil recovery methods, production profiling in such obscure completions has become a real challenge for conventional production logging operations and interpretation for log analysts. 2

3 Following sections illustrate some major challenges in highly deviated or horizontal well environment while logging with conventional production logging tools: Complex Flow Regimes In a highly deviated and horizontal well environment, several complex flow regimes exist ranging from stratified flow, plug flow, slug flow to more complicated flow regimes like annular flow, Figure-3. Using single spinner as conventional production logging tool doesn t give an insight to the real flow behavior or velocity response in the well. Figure 3: Various Flow Regimes in a Horizontal Well Environment Deviation Effect The overall velocity profile across a casing section varies with deviation. Having a single spinner reading in the middle of the casing is not true representative of actual fluid and phase velocities. The velocity may be underestimated or overestimated depending upon spinner position and orientation and depending upon what kind of flow regime may exist. Quantification of total as well as phase wise flow rates becomes significantly uncertain. Three cases are shown in Figure-4 where spinner response is affected due to amount and direction fluid flow: 1. In the first case, the areal fraction of upward flow is much higher than downward flow. Fluid velocity seen by spinner is positive. Negative velocity is not seen by spinner. 2. In the second case, downward movement of fluid is dominating the upward movement. Spinner mostly looks at negative velocity. Upward fluid movement is undetected. 3. In the third case, both positive and negative velocities are equal. Spinner which is located in the middle looks partly at positive flow and partly negative, thus gives a confusing response towards actual fluid velocity. 3

4 Figure 4: Spinner response in different fluid velocities A comparison between fluid velocity profiles in highly deviated and horizontal well is shown in Figure-5. True fluid velocity is not seen by spinner in either of two cases. Centre point measurement leads to incorrect average velocity response. Figure 6: Phase hold-up profile change with deviation in a horizontal well Figure 5: Effect on Spinner Response with change in Deviation Deviation effect becomes prominent in near horizontal wells. Significant changes in phase hold-up profile takes place with slight change in deviation in near horizontal wells. As wellbore trajectory changes its deviation from 89 to 91 degrees, heavier phase (water in this case) which was occupying almost entire cross section is now replaced by lighter phase (Oil) which covers almost whole wellbore cross-section. This is especially seen in low flow rates as shown in Figure-6: With one phase occupying the entire borehole cross section in a multiphase flow, single spinner fails to calculate true phase velocity. Hold-up measurement probes may not be able to detect the presence of second phase occupying minimum cross section inside the casing. Determine hold-up profile may not be true representative in that case. Recirculation At high heavy phase hold ups, low flow velocities, and between approximately 5 to 70 degrees deviation the phenomenon of recirculation is found. The lighter phase travels along the high side of the pipe and drags some of the heavy phase along imparting a shear to the body of the heavy phase, Figure-7. 4

5 Numerical Simulation of Recirculation (8inch ID, 1000 b/d, 85% water cut, 45degrees deviation) Figure 7: Typical Recirculation Phenomenon Observed in Low Flow, Highly Deviated wells Holdup Map Oil Holdup Velocity Map +0.5 Axial 0.0 Velocity (m/s) -0.5 Figure 8: Numerical Simulation of Recirculation Low flow rate and deviated wells with bigger size of completions, such as those in 9-5/8 casing exhibit this kind of fluid flow behaviour. A non linear velocity profile exists from the bottom to the top of the pipe with fluids travelling fastest on the high side and slowest or even downwards on the low side.under conditions such as this a spinner will average the velocities seen by the spinner blade. This average velocity may be positive or negative as discussed previously but will not be true mixture velocity.shown in Figure-8 is a holdup and velocity map generated by a numerical simulator showing the phenomenon of re-circulation. A real field example of recirculation is shown below where spinner response was completely misleading which caused inability to quantify the flow rates. Well-X, located in Mumbai offshore, India, is a deviated well with maximum deviation of 58 degrees and completed with 9-5/8 casing inside borehole. Surface test results at the time of production logging survey indicated that well was producing with high water cut (~80% WC on surface separators). All production logging sensors except spinner showed distinct signatures of hydrocarbon entry and fluid flow inside well, Figure-9. However, spinner response looked like an injection event which was essentially observed due to spinner detecting only downward flow of water which was dominating the upward flow. Figure 9: Spinner Response under Water Recirculation 5

6 Hold-Up Probes Response In A Horizontal Well Environment The basic configuration of the production logging tool consists of set of four electrical probes to calculate water hold-up and four optical probes which calculate gas holdup. Combining all probes response overall gas, oil and water hold-up profile is determined. In a horizontal well where a consistent flow regime is absent, four probes read different hold-up response and a different fluid phase depending upon position of the probe along the vertical axis of the casing. Result is incorrect phase hold-up values. Figure-10 shows how hold-up response is affected in some special fluid flow regimes observed in highly deviated or horizontal wells: Figure 10: Effect of position of hold-up probes in stratified flow regimes. In the first case above, probes measurement is limited to oil and water phases only. Gas which has a tendency to flow preferentially at the top side of the casing with high flow rate and low flow area is undetected by four probes. Hence the calculated oil and water holdups are overestimated giving rise to false interpretation. In the second case, measured gas holdup is much higher than reality. Here holdup computation indicates that 25% of casing cross section is occupied by gas and rest by water. Correlations specifically developed for highly deviated and horizontal wells provide incorrect answers under these circumstances. Operational Constraints: Tool Length Horizontal well flow profiling requires additional information in terms of better fluid velocity response and hold-up profiles. Several tool modules like DEFT and GHOST are added to the toolstring (Flagship*). Due to limitation of E-line or slickline operations in highly deviated and horizontal wells, special tool conveyance systems (MaxTRAC) are incorporated. These systems provide convenience for the tool movement in such difficult trajectories and undulating wellbores. This is turn increase the overall length of the tool string which essentially requires rig intervention and hence, precious rig time. Flow loop studies have also revealed the inefficiencies of conventional logging tools in multiphase flows. Center measurements made by such tools are inadequate for describing complex flow because the most important information is located along the vertical diameter of wellbore. Conventional tools have sensors spread out over long distances in the wellbore, making measurement of complex flow regimes even more difficult. 6

7 SOLUTION: FloScan Imager (FSI*) FloScan Imager (FSI*) is an advanced generation of tools specifically designed for highly deviated and horizontal wells which caters to fulfil the requirements and meet the challenges as discussed earlier (Figure-11). FSI* is capable of evaluating phase wise flow rates and identify complex flow regimes like stratified flow, annular flow, plug flow, slug flow, recirculation, cross-flow and many with the help of its increased number of velocity and phase sensors. Figure 11: FloScan Imager Tool The tool has a small outside diameter (OD) of 1-11/16 in. and it can be run in holes ranging from 2-1/2 in. to 9 in. using coiled tubing wireline, or MaxTRAC* well tractor system. Its short 4.9-m length makes it ideal for wells with high dogleg severity. When an even shorter tool string is desired, the 1.2-m hydraulic section used for scanning and closing the tool can be removed. Three arrays of sensors: 5 mini-spinners, 6 E-probes and 6 O-probes spread all across the wellbore. It provides velocity and holdup profile along the vertical diameter ofwellbore. Direct measurement of phase velocity and holdup profile enables accurate calculation of flowrate of different phases and reduces uncertainties involved with flow models. Figure-12 shows schematic arrangement of FSI* sensors along the vertical axis of the well: Figure 12: Comparison between a Single Spinner Production Logging Tool and FSI* Following are key features of FSI, (Figure-13): Multiphase flow profiling in highly deviated and horizontal wells for reservoir and completion evaluation. Identification of fluid and gas entries in multiphase well or liquid in gas wells. Recirculation clearly identified. Stand alone, real time, three phase flow interpretation. 7

8 Free gas velocity measurement. In a highly deviated or horizontal well, gas flows preferentially from top side of casing cross section. This often remains unseen by conventional four probes PSP* string. With the placing of two FSI* probes very close to each other at the top, gas flow measurement can be done very accurately. Flow rate quantification inside tubing with its retractable arms and non collapsible mini spinners. Determination of complex borehole flow regimes like stratified/slug/mist flow. Figure 13: Illustrates FSI response in various flow scenarios FSI* provides real time answers and helps rapid decision making for workover intervention and achievement of solutions. It also maximizes clarity of well completion performance and reservoir drainage with the help of complete scan of borehole. FSI* proves its versatility by eliminating the rig requirement or minimizing rig-time with its smaller toolstring and shorter logging operation as compared to other advanced production logging tools (Flagship*) for horizontal wells (Figure-14). Following section discusses case studies from Mumbai High Field which shows various applications of FSI* providing solution in various complicated environments. CASE STUDY - 1 Figure 14: Length comparison between FSI* and Flagship* toolstring Well-1 is a near horizontal well with maximum deviation of 85 degrees and completed in B and C layers of giant carbonate reservoir. The well was completed with 7 casing liners and 3.5 production tubing inside 8.5 borehole. Gas injection valves were installed in order to optimize the production from continuously depleting layers. The total surface production from the well was 1435 BLPD with 90% WC and total gas reported at surface was

9 SCMD (42413 SCMD of free formation gas plus injected gas). FSI* survey was carried out with objective to determine downhole wellbore flow profile and determine the gas entry points in the wellbore. Production logging was carried out inside casing and tubing section. Following observations were made (Figure-16): 1. Spinners suggested that layer C was producing some hydrocarbon with majority of water production. As shown by individual spinner response, bottom three spinners were indicating downward water flow. Recirculation was observed due to low flow rate of water and high deviation. 2. A large amount of gas and oil entry was observed from layer B as seen on top three hold-up probes. Some recirculation of water with oil bubbles was observed at bottom side of the casing. Flow regime also changed to slug flow with huge gas and oil entry. 3. Fluid velocity increased manifold as fluid entered the tubing because of reduced internal diameter. Mist flow occurred partially because of flow convergence into the tubing and partially because of the probability that additional gas was entering into the tubing from some unknown place. Further investigation of FSI* data suggested that gas rate computed inside tubing was much higher than that measured inside casing. This gas rate was close to actual total gas rate measured on surface. These results led to the suspicion that casing packer was not isolating the bottom section of the casing with the annulus between casing and tubing above packer. All the GLVs were observed to be inactive because of drawdown loss against gas lift mandrels and entire gas which was injected into the GLVs was entering from the bottom of tubing through packer leak, (Figure-17). Hence, a strong recommendation was made to replace the leaking packer in order to optimize the total production on the surface. CASE STUDY - 2 Well-2 is a horizontal well drilled with 6 borehole completed with segmented compartments with 3.5 blind and perforated tubings, sliding sleeves and swell packers. The maximum deviation of the well is o. The objective was to determine flow profile and fluid contribution across all the compartments. Multi-choke production logging was carried out in the well in order to understand well behaviour and determine layer-wise contribution. Following were the observations (Figure-18): 1. Of all contributing segments, bottom most perforated tubing section was the major oil and water contributor in both the choke sizes. 2. SS-3 and SS-4 were the major gas producing segments. 3. A small hydrocarbon entry was also observed through SS-5. On the basis of observations and results, it was suggested to close sliding sleeves SS-3 and SS-4 in order to curtail the major entry of gas. This in turn may provide additional drawdown to oil producing segment at perforated tubing section. Selective stimulation for SS-5 was also suggested. In this case, FSI* provided three phase hold-up profile along the wellbore and precisely marked the places with sudden influx of gas. This operation was successfully performed inside extreme horizontal wellbore trajectory with undulations and inside one of the advanced completions. It paved the way for new opportunities for FSI* in such kind of environment. 9

10 CASE STUDY 3 XX platform is located in southern part of the giant Mumbai High Field. Several highly deviated and horizontal wells are drilled and being produced from this platform. Wells of XX platform are mainly completed in A2-VII, N, B, C and D layers. Production logging was done in the following wells with PSP* and FSI* as shown: Well Perforated Layers Deviation (o) Job Well-3 A2-VII& B 61 FSI* Well-4 A2-VII & B 74 FSI* Well-5 B, C & D 70 FSI* Well-6 A2-VII, N & B 76 PSP* On this platform, FSI* survey was done in the wells Well-3, Well-4 and Well-5 while production logging using conventional PSP tool was also performed in Well-6. Analysis of PLT data in Well-6 clearly showed the effect of high deviation on the performance of PLT sensors. As shown in Figure-19, Spinner data was affected due to recirculation of water along the perforated layers which resulted to negative average mixture velocity. Computation for flow rate was done only at the top of A2-VII layer where spinner data was not much affected. However, hold-up probes and density response was clearly suggesting that layer N and B were also contributing hydrocarbon towards total well production which could not be quantified because of misleading spinner response. In this case, density from Gradiomanometer also was not the true representative due to its limitations beyond 60 o deviation. Overall interpretation was found to be inconclusive in this case. Production logging using FSI* on rest of the wells provided far better results than PSP*. As shown in Figure-20, 21 and 22, distinct flow characteristics were identified with the help of multi-spinner data. Recirculation in high deviation was also observed from bottom spinners. Convenience of flow rate computation inside tubing helped in understanding the behavior of well in terms of productivity apart from basic interpretation. Following section describes how FSI* data was integrated with wireline formation testing tools (MDT) with an objective to understand layer performance. Integration of Advance Production Logging (FSI*) With Wireline Formation Testing (MDT) On analyzing the FSI* data from Wells-3, 4 and 5, it revealed that layer-b was inactive wherever it was open with layer A2-VII (Well-3 and Well-4), whereas layer-b had a good production in Well-5 where it was not completed with layer A2-VII. It led to further investigation with pressure data from formation testers. MDT was recorded in these wells before initial completion; which shows a significant pressure difference (more than 200 psi) and mobility contrast between A2-VII and B layers (Figure-23). It was concluded that higher pressure and higher mobility of A2- VII layer was dominating over B-layer and thus not allowing it to produce. Following conclusions were made: 1. Dominating layer (A2-VII) subdued the production from other layers. 2. Commingle production of two layers, A2-VII and B was not a good decision. 3. Instead of commingling two layers, lower pressure layer (Layer B) could be opened first. Once depleted, layer at higher pressure (Layer A2-VII) could be opened for production. 4. Dual completion was suggested solution for simultaneous production from both layers. 10

11 CONCLUSION FSI*, the advanced production logging tool helped in the characterization of complex flow regimes and layer performance in a highly deviated and horizontal well environment in Mumbai high offshore. With its ability to perform under dynamic conditions, well problems associated with artificial lift, production and completion performance were diagnosed. Real time answers obtained during an operation helped making rapid decisions for further workover and remedial measures. Its easy conveyance inside a well with minimum restrictions up to 2-7/8 in. of tubing size opens the pathway for picking up some difficult candidate wells for production logging. Facility to quantify flow rates inside tubing makes it apt to operate in some complicated completions as mentioned above, where production logging using conventional tools was almost impossible. FSI* also eliminated the rig requirement for production logging operation with its optimum tool length. This minimizes the cost to the operators in terms of saving precious rig-time. Following table briefs some examples wells where FSI operation saved a substantial rigtime, (Figure-15): Figure 15:Operation timings for FSI* compared to Flagship* In the age of horizontal wells, production logging results must be as good as they used to be in the time of vertical and near vertical wells. FSI* is the tool for present age highly deviated and horizontal wells. Integration of dynamic downhole production logging data from FSI* with wireline formation test (MDT*) gives an insight to the layer-wise flow behaviour which can be further extrapolated for the field studies. ACKNOWLEDGMENTS The authors wish to thank Oil and Natural Gas Corporation Ltd. for permission to publish this work. REFERENCES Suryanarayana, K. And Lahiri, G. ONGC Schlumberger Wireline Research Centre, New Delhi, India, Characterization of a complex carbonate reservoir: A case study from a western offshore field of India, SPE#36193 B.E. Theron and T. Unwin, Schlumberger Cambridge Research, Stratified flow model and interpretation in horizontal wells, SPE#36560 R.D. Tewari, SPE, VAMSR Mohan Rao, SPE and A V Raju, SPE, ONGC, India, Development strategy and reservoir management of a multilayered giant offshore carbonate field, SPE#64461 S.K. Moitra and Subhash Chand, Oil & Natural Gas Corp., SantanuBarua, DejiAdenusi and VikasAgrawal, Schlumberger Data & Consulting Services, A Fieldwide integrated production model and asset management system for the Mumbai High Field, OTC

12 ABOUT THE AUTHORS Sunil Chaudhary has an M.Phil. Degree in Physics and is working as Deputy General Manager (Wells) with ONGC, Mumbai. He has played a major role in adding value to high technology petrophysical services deployed for efficient extraction of hydrocarbons. He has 27 years of experience in the exploration and development of hydrocarbons and has worked in almost all producing basins of the country. M. S. Murty obtained M.Sc. degree in Physics in 1989 from H. N. Bahuguna University, Dehradun. He joined ONGC as a graduate trainee in He has 14 years field experience of carrying out complete range of Well Logging Operations. He is currently working as a Petrophysicist in the log interpretation group of Logging services in Mumbai and also as a System and Database Manager. R. K. Pandey obtained his M.Sc. (Exploration Geophysics) from BHU, Varanasi in He worked as a research scholar at ISM, Dhanbad and Geophysicist at Gujarat Water Resources before joining ONGC in He has served in all major work Centre s of ONGC and was instrumental in setting up in-house offshore logging operations. Currently, as General Manager (Wells) at Mumbai he is responsible for interpretation and Operations of logging services. U.C. Bhatt is GM, ONGC (Logging Services). He has an experience of over 35 years in well logging operations, interpretation, contract management and planning. He is currently based in Mumbai and heading Logging Services for all Mumbai offshore logging operations. Arun Pandey is Cased hole Domain Champion for Well Integrity, Perforation and Production in Schlumberger Asia Services Limited. He has thirty years of experience with Schlumberger with different roles and job assignments. Ajit Kumar is working with Schlumberger as Senior Production Engineer in Data & Consulting Services. He s a petroleum engineering graduate from India School of Mines, Dhanbad. He has an experience of over four years in well integrity and production logging operations support. 12

13 Vibhor Verma is working as Production Engineer, Data & Consulting Services with Schlumberger Asia Services Limited. He is petroleum engineering graduate from Indian School of Mines, Dhanbad. He has been working with Schlumberger for past three years for well integrity and production logging operations support. Ravi Sinha is working as Production Engineer, Data & Consulting Services with Schlumberger Asia Services Limited. He is petroleum engineering graduate from Indian School of Mines, Dhanbad. He has been working with Schlumberger for past four years for well integrity and production logging operations support. Konark Ogra is working as Production Engineer, Data & Consulting Services with Schlumberger Asia Services Limited. He is petroleum engineering graduate from MIT, Pune. He has been working with Schlumberger for past three years for well integrity and production logging operations support. Figures: Figure 16: FSI* data interpretation for Well-1 Figure 17: Well-1 Hold-up profile along the wellbore, packer leak identified based on computed flow rates 13

14 Figure 18: Determination of Flow Profile, Flow rates and Flow Regimes by FSI* for Well-2 Spinner (rps) Pressure (psi) Density (g/cc) Temperature (degf) Hold-up from four DEFT probes Bubble Count Figure 19: Flowing passes for production logging using PSP* in Well-6, spinner indicating negative flow due to recirculation 14

15 Figure 20: FSI* flow profile and flow regime maps for Well-3, Layer A2-VII is dominating against layer B Figure 21: FSI* flow profile and flow regime maps for Well-4, Layer A2-VII is dominating against layer B Figure 22: FSI* flow profile and flow regime maps for Well-5, Layer B is major producer in absence of layer A2-VII 15

16 Figure 23: Wireline formation tester (MDT) pressure and mobility measurements for layer A2-VII and layer B showing ~200 psi pressure difference between the layers 16