Cased Hole Logging Reading Assignment. (Last Updated 16 December 2015)

Transcription

1 Cased Hole Logging Reading Assignment (Last Updated 16 December 2015)

2 NOTICE AND DISCLAIMER The information contained herein and/or these workshop/seminar proceedings (WORK) was prepared by or contributed to by various parties in support of professional continuing education. For purposes of this Disclaimer, Company Group is defined as PetroSkills, LLC.; OGCI Training, Inc.; John M. Campbell and Company; its and their parent, subsidiaries and affiliated companies; and, its and their co-lessees, partners, joint ventures, co-owners, shareholders, agents, officers, directors, employees, representatives, instructors, and contractors. Company Group takes no position as to whether any method, apparatus or product mentioned herein is or will be covered by a patent or other intellectual property. Furthermore, the information contained herein does not grant the right, by implication or otherwise, to manufacture, sell, offer for sale or use any method, apparatus or product covered by a patent or other intellectual property right; nor does it insure anyone against liability for infringement of same. Except as stated herein, COMPANY GROUP MAKES NO WARRANTIES, EXPRESS, IMPLIED, OR STATUTORY, WITH RESPECT TO THE WORK, INCLUDING, WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Company Group does not guarantee results. All interpretations using the WORK, and all recommendations based upon such interpretations, are opinion based on inferences from measurements and empirical relationships, and on assumptions, which inferences and assumptions are not infallible, and with respect to which competent specialists may differ. In addition, such interpretations, recommendations and descriptions may involve the opinion and judgment of the USER. USER has full responsibility for all interpretations, recommendations and descriptions utilizing the WORK. Company Group cannot and does not warrant the accuracy, correctness or completeness of any interpretation, recommendation or description. Under no circumstances should any interpretation, recommendation or description be relied upon as the basis for any drilling, completion, well treatment, production or other financial decision, or any procedure involving any risk to the safety of any drilling venture, drilling rig or its crew or any other individual. USER has full responsibility for all such decisions concerning other procedures relating to the drilling or production operations. Except as expressly otherwise stated herein, USER agrees that COMPANY GROUP SHALL HAVE NO LIABILITY TO USER OR TO ANY THIRD PARTY FOR ANY ORDINARY, SPECIAL, OR CONSEQUENTIAL DAMAGES OR LOSSES WHICH MIGHT ARISE DIRECTLY OR INDIRECTLY BY REASON OF USER S USE OF WORK. USER shall protect, indemnify, hold harmless and defend Company Group of and from any loss, cost, damage, or expense, including attorneys fees, arising from any claim asserted against Company Group that is in any way associated with the matters set forth in this Disclaimer. The Content may not be reproduced, distributed, sold, licensed, used to create derivative works, performed, displayed, transmitted, broadcast or otherwise exploited without the prior written content of the Company Group. Use of the WORK as a reference or manual for adult training programs is specifically reserved for PetroSkills, LLC. All rights to the WORK, including translation rights, are reserved. COPYRIGHT PETROSKILLS, LLC., 2015 THIS WORK IS COPYRIGHTED BY PETROSKILLS, LLC. AND DISTRIBUTED UNDER EXCLUSIVE LICENSE BY PETROSKILLS, LLC.

3 15. Reservoir Monitoring References - James J. Smolen, Cased hole and Production log evaluation, PennWell Books, Schlumberger, Cased Hole Log Interpretation Principles/Applications, Oilfield Review, Winter 1996, pp Introduction Reservoir monitoring is the determination and reporting of petrophysical parameters after initial conditions have been disturbed by production, e.g. to assess the effectiveness of the production (drive mechanisms). Hence, fluid distributions and phase saturations are measured as a function of time. These changes may be the result of pressure depletion, gas cap expansion, aquifer influx, water drive, etc. Hence, the monitoring of the movement of OWC and GOC fluid interfaces is an important aspect. As this is a dynamic process, many reservoir monitoring methods are time-lapse methods, i.e. they look at changes in petrophysical parameters over time. In addition, an aspect of reservoir monitoring is the determination of remaining and/or residual oil saturation (ROS). Methods The following methods exist for reservoir monitoring, each with its own advantages / disadvantages. Conventional wireline logging If open holes are available (e.g. infill wells), or the holes are not cased with steel (but with plastic, fiber glass, etc.), conventional open hole logs can be run, i.e. gamma-ray, density/neutron, induction resistivity etc. In most cases, however, such holes will not be available. Pulsed neutron capture logging This is the most commonly used method in steel cased holes. It is also known as "PNC-logging". It is best used when the formation water is rather saline. It can be run through tubing. This technique will be discussed below in more detail. Pulsed Neutron Spectroscopy logging This method is used in steel cased holes when the formation water salinity is either low or unknown. It is also known as PNS logging" or C/O logging. This technique will be discussed below in more detail. Through-casing resistivity logging PNC logs were developed because standard wireline resistivity devices could not be run in steel cased holes, as almost all current will be caught by the casing. Nevertheless, through-casing resistivity tools were developed by both Schlumberger and Baker Atlas The tool works on the principle that some current will still leak into the formation, leading to a detectable voltage

4 difference over the casing. Trials showed that meaningfull resistivity profiles were measured and that monitoring with a cased hole resistivity curve is a viable option. Both contractors now offer the tool on a commercial basis. Cross-well measurements Well-to-well acoustics can be used to delineate reservoire structure (faults etc.) between wells, and well-to-well electromagnetics can be used to detect fluid interfaces between wells. Timelapse well-to-well measurements have been used for monitoring of steam and CO 2 floods. These techniques require the availability of two open holes and are therefore expensive. The electromagnetic technique has a very low resolution and is still in the research phase. A single well deep-reading resistivity tool which is sensitive to 50 metres away from the borehole is being developed. Techniques for determining ROS include single and inter-well tracer tests, sponge coring and special core analysis. In the following the focus will be on PNC and PNS logging, being the most commonly used techniques, despite of the fact that they read rather shallowly into the formation. The assumption is that most mud filtrate invasion will have been disappeared after several months of production, so that the shallow depth of investigation is not really problematic. This is not always the case though. Moreover, because of their shallow depth of investigation these nuclear techniques are rather sensitive to the borehole and its environment (casing, tubing, cement, well treatments, etc.). Pulsed neutron capture logging ( PNC logging ) Principle In open hole logging, the hydrocarbon saturation is determined from the resistivity log. This works because the formation resistivity is sensitive to the saline water only, hence knowing the water salinity (resistivity) and the porosity the amount of water can be calculated (Archie's law). In a steel cased hole a similar technique is used. The water is again discriminated from the oil by detecting its salinity. However, in stead of detecting the saline water by means of its electrical conductivity, it is detected by using the fact that the chlorine in the salt has a high thermal neutron capture cross-section sigma, or Σ ("chlorine loves to eat neutrons"). Because of that the sigma for brine is normally (unless the water is fresh) higher than the sigma of hydrocarbons: see Fig. 1. Hence, the PNC tool is primarily a chlorine-measurement. The principle behind the PNC measurement is that sigma is relatively high in water bearing zones (because of the high chlorine content of the brine) and low in hydrocarbon bearing rock (because hydrocarbon doesn t contain chlorine). Obviously this principle only works if the formation water is sufficiently saline, i.e. if the contrast between sigma_water and sigma_hydrocarbon is sufficiently large (analogous to the fact that resistivity tools don t do a good job if the formation water salinity is too low, because the resistivity contrast between brine and hydrocarbon then disappears). The tool has an accelerator source that can be switched on and off to create a short pulse of high energy neutrons. These will be moderated (slowed down) by collisions with mainly hydrogen nuclei until they have thermal energy (this slowing down process determines the amount of thermal neutrons measured by

5 conventional open hole neutron porosity logging tools). The thermalised neutrons are captured mainly by chlorine nuclei, giving rise to gamma-radiation. These capture-induced gammas are detected by the tool. When the formation contains little oil, hence much saline water, many neutrons will be rapidly captured and hence the induced gamma-radiation will decay quickly with time. This decay is more or less exponential (Figs. 2 and 4). The slope of this decay curve is indicative for the neutron capture cross-section, called sigma or Σ (Figs. 3 & 4), which is indicative again for the water salinity (Fig. 1). Many corrections are required, however, as outlined below. Tools All major logging companies have PNC tools, all having an OD of 1 11/16". These tools all have dual detectors, the far detector being more sensitive to the formation, the near detector being used to correct for borehole influences. The differences are in the accelerators, detectors, pulse timing, measurement of the decay curve (sampling) and analysis / correction methods applied. As a further development the contractors have now tools available which combine PNS and PNC measurements. Tool OD, however, is still a limiting factor (> 1 11/16 ) for some contractors. Applications Pulsed neutron capture logs can be used in three different ways: 1) Single run (see also section on evaluation below), used to determine the absolute saturation directly from the measured sigma. The interpretation relies on a simple volumetric mixing law for the different components, being matrix, hydrocarbon, water and shale. The log measured sigma is hence a linear combination of the sigmas of the matrix, hydrocarbon, water and shale, where the coefficients are the volume fractions of these constituents (e.g. for shale it is the shale fraction): Figs. 5, 7 & 8. Hence, in order to derive the water saturation from the log one has to know the capture cross-sections (sigmas) of the matrix, hydrocarbon, water and shale. Charts and tables are available, but it is in general better to get them from the logs, e.g. by cross-plotting the log measured sigma versus porosity (Fig. 6) in clean water bearing, hydrocarbon bearing and shale intervals. The measured log (sigma) is normally displayed with the high value at the left, such that it resembles the open hole resistivity log. 2) Time-lapse logging, used to determine the change in saturation from two different runs, typically taken one year apart. This technique is much more accurate than the single run (absolute) one, and is very regularly applied. The principle is shown in Figure 9. Note that due to the time-lapse approach the only variable is the change in saturation. Uncertain parameters like sigma matrix and sigma shale have cancelled out. Figure 10 shows a time-lapse example where the rise in OWC is illustrated by an overlay of the various sigma curves acquired at different times. 3) Log-inject-log: a base log is run, next a different salinity brine is injected and the log is run again. This is used for ROS determination. Additional information Apart from the capture cross-section sigma, the PNC tool also gives the following two curves: 1. The ratio of near to far detector count rates: this parameter is indicative for porosity.

6 2. The ratio of inelastic to capture count rates for the near detector (see also section on Induced Spectroscopy below): this curve is plotted such that it overlays the curve mentioned under point 1 for oil bearing zones. It will separate to the left of the curve of point 1 in a gas zone. Hence the separation between the two curves acts as a good gas indicator. Problems - The pulsed neutron capture technique is statistical by nature. The logging speed is hampered by the fact that a sufficiently high count-rate has to be obtained. This limits also the repeatability and the accuracy of the measurements. - The formation water should be sufficiently saline, normally above 30,000 ppm, and the porosity above %, to guarantee enough signal. The accuracy in the water saturation is 5 % at best (in single run applications). Note, however, that changes in OWC can be picked-up through time-lapse mode when the formation water salinity is less than 30,000 ppm (> 20,000 ppm). - The technique relies on the fact that the neutron captures are due to chlorine. However, many other elements can capture neutrons as well, e.g. boron (which may be present in shales), potassium (in shales), iron (present in many minerals and cements). Hence, the measurement is problematic in shaly environments and in non-standard lithologies. It works best in clean sandstones and carbonates. - Neutrons are captured also rather well by the steel of the casing and tubing. Moreover neutrons are captured by the salt water in the borehole. The borehole will therefore also act as a neutron sink leading to additional apparent decay due to diffusion of neutrons away from the formation into the borehole. Only in recent years was this diffusion effect recognised to be important. These disturbances due to borehole, casing, tubing and cement make substantial corrections necessary if the tool is used in the absolute (single run) mode. Associated problems are (Fig. 11): remnant mud filtrate invasion, remnants of acid washes (HCl: chlorine rich!), wash-outs partly filled with cement, gas accumulations just below the packer (between tubing and casing), etc. - Because of its shallow depth of investigation the inferred fluid contacts can be misleading in cases of water or gas coning. - Because of the above the tool is best used in the time-lapse mode. Even in this mode it is only accurate if the only change happening is the one in the formation due to the production. Changes in borehole fluid, borehole environment (sand production, acid treatment, etc.) and tool type should be avoided. Pulsed Neutron Spectroscopy Principle Another way to detect hydrocarbons is the measurement of the ratio of carbon to oxygen atoms, because this ratio will be high for hydrocarbons and low for water (Fig. 12). This ratio is measured by again using pulsed neutrons, but now looking not at the thermal decay, but at the inelastic scattering immediately after the burst. The neutrons then still have a high energy. Inelastic collisions with nuclei will generate gamma rays whose energies are specific to the element that is hit by the

7 neutron. Hence the measured energy spectrum of the thus induced gammas is a kind of fingerprint for the chemical composition (a different version of the tool is used for the determination of lithology). Special energy windows are used to separately measure the amount of carbon and the amount of oxygen. This technique basically measures the relative abundance of hydrocarbon with respect to water and is therefore salinity independent. Hence, it can be used in cases where pulsed neutron capture logging doesn't work, e.g. CO 2 flooding where comingled salinities are present. Apart from inelastically scattered gamma-rays (which occur very soon after the neutron pulse), also gamma-rays from thermal neutron capture will be measured (these occur later in time). From all of this information not only information on Carbon and Oxygen is obtained, but also information on other elements such as Silicium, Iron, Calcium, Sulphur, Chlorine etc. Herefrom an indication of lithology can often be obtained. Tools Most contractors offer combined PNS and PNC services. Note, however, that some contractors can not provide a 1 11/16 tool. Applications PNS logging has been successfully used to identify formation fluid types in low salinity reservoirs. Even remaining oil behind tubing has been identified with PNS logging.

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9 Fig. 1 Neutron capture cross-sections Boron 45,000 per g/cc Chlorine 570 Hydrogen 200 Silicon 3 Matrix 8-12 c.u. Oil Gas 8 Pure water 22 Saline water Shale Borax 9,000 Rock salt (halite) 726 Iron minerals (pyrite etc.) Fig. 2 Neutron life time <---> Chlorine Neutron faces few capturers Neutron faces many capturers neutron Cl Cl neutron Cl Cl Cl Counts slow decay: low Σ Counts fast decay: high Σ time time

10 Fig. 3 Formation capture cross-section Σ Number of neutrons N versus time t: dn = N Σ vdt N = N 0 exp(- Σvt) = N 0 exp(- t / τ) v = thermal neutron velocity Σ = formation capture cross-section (slope of decay curve on log-log plot) Σ = 4550 / τ τ = decay time constant (μs) Influence of pore fluid on decay curve 1000 Region of borehole and casing decay Relative count-rate Saline water Oil Gas Region of formation decay Background 1 Time (μs) after neutron burst Fig. 4 Effect of Pore Fluid on the Decay

11 Fig. 5 TDT response in clean sand Σ = (1 - φ) Σ ma + φs w Σ w + φ(1-s w )Σ hc Σ ma and Σ w can be obtained from regression through water bearing points Σ hc (and possibly Σ ma and Σ w ) can be obtained from charts / lab. measurements Σ Fig. 6 TDT response in clean reservoir Water bearing zone Water line Σ w S w Σ m 0 Σ Hydrocarbon line hc Oil bearing zone Porosity

12 Fig. 7 TDT response in HC sand log: matrix hydrocarbon water shale Σ log = (1 - V sh - φ)σ ma + φ(1 - S w )Σ hc + φs w Σ w + V sh Σ sh = ( )* *(1-0.2)* *0.2* *25 = 0.6 * * * *25 = = 16.6 Fig. 8 TDT sigma interpretation Σ = (1 φ) Σ ma +φ Σ fl (macroscopic volumetrics) Σ fl = S w *Σ w +(1 S w ) Σ hc Σ ma, Σ w, Σ hc from Σ vs. φ cross-plot, or from charts / lab. Additional curves: - near/far ratio: pseudo CNL porosity - near/far overlay: gas & shale discr.

13 Fig.9 Time-Lapse Difference between a base run and subsequent runs is: ΔΣ log = φ (Σ w - Σ hc ) ΔS w Hence: Σ ma and Σ sh have dropped out of the equation. Thus: ΔS w = ΔΣ log φ (Σ w - Σ hc ) Fig. 10 Far East Carbonates PNC data: Moving Contact & Change in Residual Gas Saturation Residual gas zone (swept by water front) Paleo-residual gas zone (gas expanding w/ pressure decline) Water saturated zone

14 Fig.11 Influence of BH environment Use well sketch besides PNC logs cement remaining invasion formation HCl acid wash, washouts,... Fig. 12 Low Salinity Environment.Pulsed Neutron Spectroscopy Inelastic scattering Neutron is scattered Fast neutron (14 MeV) Nucleus Emitted gamma-ray Excited nucleus Hydrocarbon C x H 2x+2 carbon rich no oxygen Water H 2 O no carbon oxygen rich