and their inclusion in the planning database are currently under field test. ABSTRACT

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1 Effective Pulsed-Neutron Logging in both Tubing and Casing for Brown Fields By M L Sanni, S McFadden, Shell Pet Dev Co Nigeria, S Kimminau Shell International, P Wanjau, L. Silipigno, Schlumberger Sugar Land, B. Roscoe, Schlumberger Ridgefield. ABSTRACT Reliable logging in both tubing and casing in Niger-Delta oilfields has allowed bypassed oil to be found and produced for very low costs, typically less than 1 dollar per barrel. Fields in the Niger-Delta often consist of a stack of many completable reservoirs, each seldom more than a few hundreds of feet thick, but together stretching over several thousands of feet vertically. In most completions many potential reservoir intervals are behind both casing or liner and at least one string of tubing. Cased-hole logging provides information on fluid contact movements and thus reservoir connectivity during the production life; it has identified bypassed oil previously thought to have been developed by existing producers and, conversely, unperforated sands being drained via neighboring reservoirs. Carbon-oxygen logging through both casing and tubing became possible in 1991 with the introduction of a 1 11/16 in. OD tool. However, the tool was originally intended to be conveyed through tubing on its way to an interval with a single casing or liner below the tubing shoe, and it had not yet been characterized for logging inside two steel tubulars. Initial field trials inside two tubulars concentrated on establishing fluid contacts on an empirical basis, while a research program provided both Monte- Carlo modeling data and a laboratory characterization database. Laboratory data have been acquired in 68 conditions, characterizing tubing sizes of 2 3/8 and 3.5 in. within 8.5 and 12 in. boreholes. A software job planning tool calculates carbon-oxygen yields and statistical precision under a variety of completion conditions, allowing the optimal data acquisition program to be planned. Interpolation between different completion cases is not automated because of the sparsity of the measurements and the complexity of the completion geometry, but a library of well-characterized cases allows intercomparison. Field results have been encouraging, with more than 70 percent of cases clearly indicating fluid contacts that have subsequently been proven by recompletion. The new characterizations and their inclusion in the planning database are currently under field test. 1

2 INTRODUCTION Pulsed-neutron logging is today the primary means of identifying unswept and bypassed hydrocarbons in producing oilfields all over the world. The theory of operation of such devices is explained in detail in standard texts (1). Two basic elements are critical to the successful application of this technique in the Niger-Delta: the availability of a 1-11/16-in. OD carbon-oxygen log, and the response characterization for through-tubing-and-casing conditions. Pulsed neutron logs fall into two main categories; pulsed-neutron capture, or PNC logs, which have been available for many years in 1 11/16-in. (42mm) OD, but which suffer from reduced sensitivity in low or variable water-salinity environments, and pulsed-neutron spectrometry, or PNS logs, which measure the carbon-cxygen (C/O ratio) and are insensitive to water salinity, but due to technical reasons have not historically been available in diameters smaller than about 3 inches. In 1991 a new PNS carbon-oxygen tool became available (2,3) in 1 11/16-in. OD, making logging through both casing and tubing possible. However, the tool was originally intended to be conveyed through tubing on its way to an interval with a single casing or liner below the tubing shoe, and it had not yet been characterized for logging inside two steel tubulars. PNC logs are relatively insensitive to the steel borehole contents because they respond mainly to the thermal neutron capture die-away time in the rock formation surrounding the borehole. The borehole effects are largely compensated by a time-based processing system (4). PNS logs on the other hand record a spectrum which comes simultaneously from both the borehole and the formation, and respond to carbon and oxygen atoms in both regions. Effective pulsed-neutron logging through tubing and casing in the Niger Delta requires a tool capable of both PNC and PNS carbon-oxygen measurements, packaged in 1 11/16-in. OD, posessing accurate characterization of the tool response in these special conditions so that formation carbon content, and hence oil saturation, can be determined accurately enough to unambiguously detect unswept oil, or conversely to show its absence. RESERVOIR CONNECTIVITY IN THE NIGER DELTA Typical Niger-Delta oilfields consist of a stack of many completable reservoirs, each seldom more than a few hundreds of feet thick, but together stretching over several thousands of feet vertically. In most completions many potential reservoir intervals are behind both casing or liner and at least one string of tubing. If reservoir connectivity were reliably known, location of unswept reserves would be a much easier task, for example evidence of water production at an up-dip sand would provide conclusive evidence that down-dip sands had already been swept. Conversely evidence that a down-dip sand is still oil-bearing leads to the conclusion that up-dip sands are still unswept and are targets for recompletion. Without absolutely reliable reservoir connectivity data, a very large 2

3 uncertainty exists in knowledge of reservoir drainage patterns. Cased-hole logging provides information on fluid contacts in each well logged. Measurements made over time reveal movements and thus reservoir connectivity during the production life. Accurate and reliable C (- Contact) data can identify bypassed oil previously thought to have been developed by existing producers and, conversely, unperforated sands being drained via neighboring reservoirs. The Niger-Delta examples show both cases, allowing recovery of previously unswept oil, and also avoidance of water-swept zones. Figure 1 illustrates a case in which a thin shale separates two sands, but it is continuous and impermeable enough to prevent vertical sweep between the two sands. Figure 2 illustrates another case, in which the lower sand is in vertical communication with the upper sand (which is being produced in another well), although in this case the shale is thicker, and might have been thought to be an effective seal. SPECIAL CHARACTERISATION Some log measurements (such as resistivity) have responses which can be calculated for many environments by the use of precise and completely characterized physical models. Nuclear logs in general, and PNS logs in particular, must be characterized for a given environment, typically through laboratory measurements. Useful characterization data can also be obtained through nuclear Monte-Carlo modeling methods, although some difficulties remain with the absolute accuracy of prediction of this technique. Several factors contribute to inaccuracies, the most important of which is the problem of modeling the fine details of detector response. The effectiveness of Monte-Carlo can be greatly improved if benchmark laboratory data is available, that is data sets which are measured sufficiently close in completion parameter space to assume an acceptable degree of similarity across typical porosity variations encountered in a well. Why then is the through-tubing-andcasing situation so difficult to characterize? One simple reason is the fact that two borehole fluids, or even mixtures, are now involved; the production tubing contents and the tubing-casing annulus contents. Carbonoxygen logs have different sensitivities to fluids in the different regions. Another complication is that the tubing may be centered or eccentered within the casing. In some cases two tubing strings are present in the casing. Benchmark carbon-oxygen measurements through both casing and tubing were carried out at the Schlumberger Environmental Effects Calibration Facility (EECF), Houston, Texas. Two typical completions were identified and constructed for use with existing EECF neutron tank formations. Figure 4 shows a side-view schematic of a single tubing inside the casing in an EECF neutron tank, with the 1 11/16-in. OD carbon-oxygen tool inside the tubing. The tool was eccentered in the tubing during all measurements. For the single tubing measurements, the tubing was positioned both centered and eccentered 3

4 in the casing. For the dual tubing measurements, the tubing axes were positioned at the 1/3 and 2/3 points of the inner casing diameter. Measurements were made in a total of 68 combinations of fluids and tubing positions. These are summarized in Table 1, and the computed carbon/oxygen ratios are plotted in Figures 5 to 8. The fluid endpoints represented in the figures are: - water in borehole and water in the formation, - oil in borehole and water in the formation, - water in the borehole and oil in the formation, and - oil in the borehole and oil in the formation. PNS RESPONSE IN BOTH TUBING AND CASING Figures 5 and 6 show results obtained in a single 2 3/8-in. tubing within a 7-in. casing. The two rhomboids labeled No Tubing are the same in each diagram and are the same as are published in standard response charts. They serve as a reference for the other two cases in each figure, which show the effect of centered and eccentered tubing. As one might expect, the tubing measurements show a reduced carbon sensitivity in oilfilled tubing (Figure 6). It is also interesting to note that the extra tubing steel around the tool produces a noticeable offset in the zero-carbon (water-water) case. Figures 6 and 7 show the corresponding results for 3 1/2-in. tubing in 9 5/8-in. casing. Again, the two rhomboids labeled No Tubing are the same in each diagram and are the same as are published in standard response charts, serving as a reference for the other two cases. As expected, the environmental effects are much larger than in the case of smaller tubing and casing. Whilst the responses seen in Figures 5 and 6 could be re-normalized if sands with known or inferred saturation are used as a reference, it can be seen that these effects in larger diameters imply significant changes in response. Response rhomboids such as these have previously been published in response charts, (only for the single-casing situation), but a new software job planning tool calculates carbon-oxygen yields and statistical precision under a variety of completion conditions, allowing the optimal data acquisition program to be planned. For the multipletubing completion geometry, interpolation between different completion cases is not automated because of the sparsity of the measurements and the complexity of the completion geometry, but a library of well-characterized cases allows intercomparison. NIGER-DELTA APPLICATION AND RESULTS Figure 3 shows analysis of a carbonoxygen log in a typical Niger-Delta well with two sands presented within the logged interval. These sands were interpreted from open hole logs to share a common original C close to the bottom of the lower sand. An RST-A log run in both 2 3/8-in. tubing and 7-in. casing has been interpreted to show that two different current - contacts are present, and hence that the shale between the sands is acting as a vertical flow barrier. Note that although the characterization is not yet perfect (mismatches can be seen between the 4

5 original saturation in unswept zones), it is nevertheless quite fit-for-purpose to achieve the objective of locating unswept oil, especially with regard to reperforation. CONCLUSIONS Application of pulsed neutron hydrocarbon-water contact logging in Niger-Delta reservoirs has been encouraging, with more than 70 percent of cases clearly indicating fluid contacts that have subsequently been proven by re-completion. Finding and production costs are very low, typically less than 1 dollar per barrel. Through tubing and casing carbonoxygen log laboratory data have been acquired in 68 conditions, characterizing tubing sizes of 2 3/8 and 3.5-in. within 8.5 and 12 in. cased boreholes, respectively. The new characterizations and their inclusion in the planning database are currently under field test. ACKNLEDGMENTS services for permission to publish the log examples and RST characterization data. Personal thanks are due to Mr Ton Loermans of SPDC who initiated and supported much of the work, and to professor Max Peeters, formerly of Shell, and now professor of petrophysics at Colorado School of Mines. Mark of Schlumberger REFERENCES 1. Well Logging for Earth Scientists, Chapter 13- Pulsed Neutron Devices, pp , Darwin V Ellis, Elsevier New York A New Through-Tubing -Saturation Measurement System, B A Roscoe, R A Adolph, Y Boutemy, J C Cheeseborough, J S Hall, D C McKeon, D Pittman, B Seeman, and S R Thomas, paper SPE presented at the SPE Middle East Show, Bahrain, November A New Compensated Through-Tubing Carbon/Oxygen Tool for use in Flowing Wells, H D Scott, C Stoller, B A Roscoe, R E Plasek, and R A Adolph, paper MM, presented at the SPWLA 32 nd Annual Logging Symposium, June 16-19, Improved Pulsed Neutron Capture Logging With Slim Carbon-Oxygen Tools: Methodology, R E Plasek, R A Adolph, C Stoller, D J Willis, E E Bordon and M G Portal, paper SPE presented at the SPE Annual Technical Conference and exhibition, Dallas, USA, October The authors wish to thank Shell Petroleum Development Company of Nigeria and Schlumberger field 5

6 Table 1: LABORATORY MEASUREMENT LISTING Configuation 1 8 ½ in. Borehole / 7 in. Casing / No Tubing (33 p.u. Sandstone Formation) Configuation 2 8 ½ in. Borehole / 7 in. Casing / Single 2 3/8 in. Tubing (33 p.u. Sandstone Formation) Configuation 3 8 ½ in. Borehole / 7 in. Casing / Dual 2 3/8 in. Tubing (33 p.u. Sandstone Formation) Configuation 4 12 in. Borehole / 9 5/8 in. Casing / No Tubing (33 p.u. Sandstone Formation) Configuation 5 12 in. Borehole / 9 5/8 in. Casing / Single 3 ½ in. Tubing (33 p.u. Sandstone Formation) The tool is positioned eccentered in tubing 1 throughout Note: Fresh water, 0 kppm Config Tubing 1 Positions Tubing 1 Fluids Tubing 2 Position Tubing 2 Fluids Borehole Fluids 1 Formation Fluids 1 2 CEN 2 ECC 3 3 1/3 4 2/ CEN 2 ECC 3 1) When tubings are present, Borehole refers to tubing - casing annulus 2) CEN = Centralized 3) ECC = Eccentralized 4) In the dual tubing configurations, each tubing axis is positioned at 1/3 and 2/3 of the inner casing diameter Air Air 1 11/16 OD C/O Tool Eccentered in Tubing Single Tubing Eccentered in Casing Formation Flush Formation Material Casing and Cement Sheath Formation Drain Spool Borehole Fill & Drain Rathole Fig. 4 Neutron Laboratory Measurement Formation Schematic 6

7 Sand 1 Sand 2 Original C Thin Shale Separates Sands 1 & 2 Sand 1 Sand 2 Different Cs Figure 1. Thin shale between sands 1 & 2 does act as a flow barrier, PNS log in a well shows different Cs due to production. Sand I Sand II Original C Thick Shale Separates Sands I & II Sand I Sand II Same Cs Figure 2. Thick shale between sands I & II surprisingly does not act as a flow barrier, PNS log shows same new Cs. 7

8 Measured Carbon/Oxygen Ratio Response in Single 2 3/8" filled Tubing. 8 1/2 in. Borehole, 7 in. Casing, 33 p.u. Sandstone Formation Far Detector Carbon/Oxygen Ratio No Tubing 0 Eccentered Tubing Centered Tubing Near Detector Carbon/Oxygen Ratio Fig. 5: Carbon/Oxygen response in single 2 3/8 in. water filled tubing, 7 in. casing, 8 ½ in. hole Measured Carbon/Oxygen Ratio Response in Single 2 3/8" filled Tubing. 8 1/2 in. Borehole, 7 in. Casing, 33 p.u. Sandstone Formation Far Detector Carbon/Oxygen Ratio No Tubing 0 Eccentered Tubing Centered Tubing Near Detector Carbon/Oxygen Ratio Fig. 6: Carbon/Oxygen response in single 2 3/8 in. oil filled tubing, 7 in. casing, 8 ½ in. hole 8

9 Measured Carbon/Oxygen Ratio Response in Single 3 1/2" filled Tubing. 12 in. Borehole, 9 5/8 in. Casing, 33 p.u. Sandstone Formation Far Detector Carbon/Oxygen Ratio No Tubing 0 Centered Tubing Eccentered Tubing Near Detector Carbon/Oxygen Ratio Fig. 7: Carbon/Oxygen response in single 3 1/2 in. water filled tubing, 9 5/8 in. casing, 12 in. hole 1.4 Measured Carbon/Oxygen Ratio Response in Single 3 1/2" filled Tubing. 12 in. Borehole, 9 5/8 in. Casing, 33 p.u. Sandstone Formation 1.2 Far Detector Carbon/Oxygen Ratio No Tubing 0 Centered Tubing Eccentered Tubing Near Detector Carbon/Oxygen Ratio Fig. 8: Carbon/Oxygen response in single 3 1/2 in. oil filled tubing, 9 5/8 in. casing, 12 in. hole 9