Advanced Reservoir Monitoring

Transcription

1 Advanced Reservoir Monitoring Water Oil

2 Overview Following topics will be addressed in this course: Saturations of Water (Sw), oil (So) and gas (Sg) are likely to change with time. This is caused by fluid movements in the reservoir caused by production and gas and water injection as part of secondary and tertiary recovery. The saturation changes are not uniform, and are affected by permeabilities, structural factors such as faults, and localised barriers such as shale beds and tight reservoirs. The computations of saturations are complicated by factors such as low salinities, unknown salinities and low porosities which limits the resolution. Near wellbore conditions, such as multiple casings, damaged zones and enlarged borehole can add to the complexity of evaluations. p-2

3 Reservoir Monitoring Challenge Changes in Water/oil/gas saturations during production: This is the primary objective of saturation monitoring and to obtain that as a function of time. Variations in the sweep water salinity: The values of water salinity are important for estimating Sw from Pulsed Neutron Log (PNL) capture mode. This is often unknown as the water composition is a cocktail of formation water and the different injection waters Pulsed Neutron Log (PNL) Applications (capture and inelastic modes): PNL is the main log used to estimate SW and Sg. Both modes needs fine tuning of the interpretation parameters. Resistivity measurements behind Conductive and non-conductive casings: Ideally resistivity measurements (Rt) can be of great help as a direct comparison can be made with original open hole Rt.. Two tools are developed to measure Rt in conductive and non-conductive casings. Deep Reading Electromagnetic Imaging (EM): Most logging tools have a limited depth of investigations (<4 m). Well to well EM can help to image water sweep few hundred meters between wells.. Field wide water sweep mapping: The ultimate objective is to map the water flood on a field wide basis for each selected zone.. Log-Inject-Log for residual Oil Saturation (Sor) and Gravel Pack monitoring: PNL logs can be used to evaluate for Sor very accurately and to evaluate the gravel pack quality Fluid sampling: A dynamic tester is used to obtain fluid samples fluid samples and reservoir pressures behind casings. p-3

4 Fig-1 Pulsed Neutron Logging (PNL) Fig-2 Fig-3 Fig-4 Pulsed Neutron Logging (PNL): PNL is the backbone of saturation monitoring. PNL is made up as follows: A neutron generator emitting 20 million neutrons at an energy of 17 MeV (Fig-1). The neutron are slowed down as they collide with the various atoms (Fig-2). At high speed the atoms hit by the neutrons emit spectroscopy (Fig-3), this is the basis of Carbon/Oxygen logging. Eventually the neutron slow down to thermal level and get captured (Fig-4). This is the basis for capture mode (Σ) p-4

5 3-D image of PNL Process GR Count GR Count Pulsed Neutron Logging (PNL) Modelling: The image above is a 3-D model outlining the PNL process. Y-axis: are the count rates for the neutrons and Gamma Ray spectroscopy Z-Axis is the energy of the emitted GR caused by inelastic interaction. X-axis: Is time Normally the emitted neutron time life is about 200 micro-sec. In the first 50 micro-sec, inelastic interaction takes place and the results are shown as blue waveforms; The Y-Z plot is shown above In the last 100 micro-sec, capture modes takes place. This is shown as black waveforms. The capture-time plot is also shown above. p-5

6 PNL Capture Mode to Estimate Sw PNL1986 PNL1983 OH 1980 PNL Capture Mode: The figure on the right shows the neutron capture as a function of time for various Sw values. This is used to obtain Sw values The figure on the left shows the results of open hole logs and 2 timelapse PNL capture mode results. This is used to estimate water saturation changes (shown in dark blue) as a function of time. p-6

7 PNL Capture Mode to Estimate Sg PNL Capture Mode to estimate Sg: The figures above shows 2 time lapse logs used to monitor the expansion of the gas cap The data is accurate with an error range of 10% for porosities >= 15%.. For lower porosities, only qualitative estimation can be made. In this example, the expanding gas cap is assumed to replace the oil leaving the irreducible water saturation unchanged. p-7

8 Stand-alone PNL Capture Mode interpretations in old wells with no previous data Stand-alone PNL interpretations: There is a large number of wells that were drilled and produced with very limited amount of data. It is estimated that more than 50% of wells in the Middle East fit this category. A stand-alone PNL log can be used to provide reasonable formation evaluation to analyse old reservoirs for missed hydrocarbon zones: The PNL log can give GR, Porosity and capture (Σ) The GR is used to estimate Vsh. The effective porosity is computed from the neutron porosity using an average of laminated/dispersed shale models. The water salinity is obtained assuming that there is a flushed zone with a given Sro. The estimated Sw log is shown above on the right. p-8

9 PNL Inelastic Carbon/Oxygen (C/O) Mode Inelastic C/O: The top figure shows a classical spectroscopy obtained from elemental GR emissions. Since the inelastic mode takes place in the early life of neutrons, a significant part of this spectrum is obtained from the wellbore fluids. A family of empirical trapizums are obtained to take into considerations borehole conditions (casing size, borehole size, borehole fluids, porosity, lithology, etc..). These trapeziums are then used to obtain the values of SW, independent of water salinity. p-9

10 PNL Inelastic Carbon/Oxygen (C/O) Mode Inelastic C/O: Field Example A field example of C/O application The original open hole water saturation includes the green and black areas. A C/O log was run 8 years later. The green area is the depletion evaluated from this log. Below X360 there is either water zone or a residual oil zone. The C/O data and the OH data show a perfect fit over this bottom interval. p-10

11 PNL Capture and Inelastic modes used to obtain Sw and volume of injected water. Track-1 Track-2 Track-3 Variable Flood: Field Example If we run both Σ and a C/O log we can solve for two unknowns: Sw and the value of Σwater. The Σwater value can then be used to differentiate between injected water and formation water. The example above shows 3 interpreted data: T1: Original Open Hole saturations T2 Sw from C/O T3: The volumes of injected water (purple) and formation ware (blue) This is used to monitor areas where the injection water has by-passed. p-11

12 Rt through Conductive Casing Cased Hole Formation Resistance: Ideally, obtaining time-lapse Rt values is the easiest way to compare changes of Rt from the original OH Rt Recent technology developments made that possible. This involves sending an axial current in the casing and estimating the leakage current over two successive intervals. This essentially assumes that we have two resistances in parallel; Casing resistance Formation resistance The real challenge is that the leakage current results in a change of voltage of the order of 5*10-9 volts between two successive monitoring intervals. The example above is very informative. No depletion over the top perforations. p-12

13 Rt through Non-Conductive Casing Cased Hole Formation Resistance- non-conductive casing: The induction resistivity log is designed to operate where the medium around the tool is non-conducting. In open hole this is ideal for oil-base mud. In some wells plastic casings or fibre-glass casing are used. This is done for monitoring wells. In other cases the well is completed as bare-foot, with oil or gas in the borehole. A slim-hole induction log is designed for this purpose. The induction log is ideal for such data acquisition. p-13

14 X-Well Deep reading Electromagnetic Imaging X-Well Deep reading Electromagnetic Imaging: This technology is developed to have deeper readings and hence deeper imaging of water flooding. A series of transmitters and receivers are placed on two wells. The position of these receivers/transmitters are changed and a large amount of data are obtained. Data inversion is done to determine the fluid composition between the 2 wells. The example above is for 2 wells 150 apart. Good results were also obtained for wells 1000 ft apart. p-14

15 Cased Hole Dynamic Tester Tool Sketch Drilling process Sampling process Cased Hole Dynamic Tester: In few cases log data may not be conclusive. A tool designed to drill a hole in the casing, do a pre-test and take fluid samples was designed and operated successfully. This tool has many applications: For old wells where some of the reservoirs were not tested Identifying un-swept hydrocarbon zones Water sampling to determine the source of water The drilled hole is plugged on completion of the job by the same tool. p-15

16 Log-Inject-Log and Gravel-Pack Monitoring Log-Inject-Log (LIL) Gravel-Pack Monitoring Log-Inject-Log and Gravel-Pack Monitoring: The PNL capture mode log can also be used to do other measurements. Two important measurements are: 1- Log-Inject Log High salinity water is injected in the formation at a low rate, and successive PNL Σ logs are obtained. The value of logged Σ will increase gradually as the water sweeps any moved hydrocarbon. At the maximum and stable value of Σ the value of Sro can be estimated. 2- Gravel Pack monitoring The gravel is made of Si and Al. When these two elements are bombarded with neutrons, they are nuclear activated acting as a GR source with a half life of 2.3 minutes. A GR tool run after the source will measure this gamma ray emission. Any gaps in the gravel will appear as low values of gamma ray. p-16

17 Agenda Day 1 Reservoir fluids Reservoir drive mechanisms Inflow and outflow performance Justifying running reservoir monitoring logs Day 2 Nuclear physics of reservoir monitoring and pulsed neutron logging (PNL) PNL tools and scintillation detectors PNL for Capture cross section measurements Day 3 PNL for Carbon/Oxygen (C/O) logging applications Combined capture and inelastic modes to monitor injection water sweep. Log-Inject-Log to estimate Residual Oil Saturation Gravel pack monitoring Day 4 Stand-alone PNL data acquisition and interpretations in wells with limited data. Cased Hole Formation Resistivity behind steel casing Formation resistivity behind non-conductive casing Pressure measurements and sampling behind casing. Day 5 Field mapping of water flood to identify unswept zones X-well electromagnetic imaging There will be daily practical workshops on each of the topics covered using field examples p-17