The project successfully used cased hole resistivity measurements to identify, isolate and access behind-flood-front oil in wells watered out as much as 10 years ago. The ‘A3-A6’ reservoir at Elk Hills has undergone peripheral waterflood for 20 years and has a Dykstra-Parsons permeability variation of ~ 0.7. Reserve development now targets bypassed oil. In the past, determination of flood front position required open hole resistivity logs. Traditional cased-hole logs have been inconclusive as flood front monitoring tools. Only resistivity measurements provide the direct measurement to discern oil and water in this reservoir. Watered out wells are mechanically prepped and then logged with a cased hole resistivity tool. Separation between the original and present resistivity curves indicates swept sands. When the open hole and cased hole resistivity curves overlay in sands, it gives a positive indication of unprocessed, bypassed oil reserves. From this work, the ‘A3’ sand has been identified as unprocessed by the waterflood in several wells and processed in another well. Isolation and reperforation has resulted in commercial production in three wells to date. Behind casing resistivity measurement is the only logging method to successfully discern oil and water in the ‘A3-A6’ waterflood. Every wet well is now reviewed as a candidate for this surveillance technology. Results demonstrate that behind-casing resistivity measurement is a next generation waterflood surveillance tool. Introduction The ‘A3-A6’ reservoir is located on the Northwest Stevens structure in the Elk Hills field near Bakersfield, California (Fig. 1). The ‘A3-A6’ reservoir has been undergoing peripheral waterflooding since 1983.The reservoir is a series of turbidite sands with more than 200 ft of vertical thickness. These sands are grouped by increasing depth with the following nomenclature: ‘A3’, ‘A4’, ‘A5’, and ‘A6’ sands. Figure 2 is structure contoured on one of the ‘A6’ sands showing over 800 ft of relief. This structure map shows the position of the injectors and the 90% water cut line as of January 2002. The map highlights the position of three wells behind the flood front in which the new through-casing resistivity logging tool was run to identify bypassed oil. Success in the use of cased hole resistivity measurements to find and isolate behind-flood-front oil in a maturing waterflood is demonstrated. Reservoir Description The ‘A3-A6’ reservoir is located on the western limb of the Northwest Stevens anticline in the Elk Hills field. The ‘A3-A6’ sands are a series of turbidite channel sands of Miocene age pinching out toward the east. There are four major productive units ranging in depth from approximately −7,600 ft SS to −8,550 ft SS, covering an area of approximately 1,300 acres. The reservoir dips 30°-40° along the flanks of the anticline and broadens and flattens slightly moving westward down the anticlinal plunge.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractContinuous improvement measured in time savings required to sidetrack and redrill 133 shallow wells is documented. Time savings and a learning curve are related to decision making through a catalogue of sequential, applied techniques. Paralleling the evolution of applied techniques at the wellsite, bit and reamer technology evolves. Steer and ream while drilling tools (SRWD) evolve and make the sidetrack program viable. The confluence of geology, reservoir character as well as existing/new technology and management philosophy results in forecastable, positive changes. The incremental improvements produce a multi-year sidetrack program to successfully exploit the mature Elk Hills oil field.
The Main Body 'B' reservoir in the Elk Hills Field is a peripherally waterflooded, +400' thick series of layered, turbidite Stevens sands. Permeability variation between layers adversely affects the vertical sweep, resulting in production from lower permeability oil sands dominated by production from higher permeability sands. This paper discusses the unique use of various tools to identify water zones to isolate and oil zones to stimulate. Tools used to identify water and oil entry are discussed With respect to their capabilities of identifying oil and water entry into the well bore. Swept and unswept layers are identified and the confidence level to propose a water isolation remedial has been developed; The confidence level is based on the analysis of a video camera log, open hole logs, production logs and production tests. This analysis begins when a candidate well with a high water cut is shut in, and the afterflow is viewed with a video camera. Oil entry is identified and evidence of water entry is inferred. These observations are compared with results from production tests in offsets, electric logs showing processed layers In an offset redrill, and production logs, and a video camera log from the same well. Agreement in the data provides the confidence level regarding what zones to isolate as watered-out and what zones will produce economic oil rates after the remedial. If results from water isolation remedials are available, these results may also be reported. Introduction New tools are required to identify oil and water entry to propose economic water isolation remedials in high rate, high water cut wells. Specifically the tools must identify oil entry with a high level of certainty and detail. This level of certainty and detail is required in reservoirs that represent an economically risky environment with respect to water isolation remedials. Prior to using the video camera log, the primary tools available included offset production tests and production logs. Yet these conventional methods do not satisfy the degree of certainty required in this high risk environment. The Main Body 'B' reservoir at Elk Hills is characterized by heterogeneities between individual sand lobes and energy from a maturing, peripheral waterflood. These aspects create a challenging environment for water isolation due to 1) uncertainty of the injection water advancement by layer, 2) high reservoir pressure resulting in additional remedial costs, and 3) high reservoir pressure resulting in an increased likelihood of isolation failure due to high pressure differentials after the isolated layers undergo drawdown differentials. The performance of the Main Body 'B' reservoir indicates uneven flood front distributions, particularly with respect to vertical conformance. P. 383
This paper summarizes and contrasts the peripheral waterflood performance of two reservoirs that constitute the 31S project in the Elk Hills Field, Kern County, California. The combination of light oil (36 API), favorable mobility ratio (0.4) and an anticlinal structure makes these sandstones good candidates for peripheral flooding. From 1978 until December 1995 cumulative water injection in the 31S project was 371 million barrels and cumulative recovery was 155 million barrels of oil. Early initiation of a pressure maintenance project including 117 BCF crestal gas injection has improved oil recovery. The Main Body 'B' reservoir is exhibiting a distinctly layered response with high recovery from its more permeable layers and lower recovery from tighter layers. This flood has the potential for bypassed reserves. The Western 31S reservoir has moderate to high dips and is exhibiting a less layered flood response and very efficient displacement. The gravity drainage component from the secondary gas cap is playing a significant role in the process. Aside from high recovery, the project is remarkable in demonstrating successful resaturation of a secondary gas cap. Future work will include refinement of original-oil-in-place (OOIP) estimates with a new petrophysical model and improving tight layer management by pattern waterflooding and increasing productivity and injectivity with stimulation treatments. Introduction In the literature there are a few papers describing peripheral waterfloods. The 31S project in the Elk Hills Field, California provides an opportunity to study a large, mature peripheral waterflood and to see the effects of a long term injection into two very different reservoirs. The Elk Hills Field is located about 20 miles WSW of Bakersfield, California (Fig. 1). It is one of the 10 most productive oil fields in the United States with a cumulative recovery of over one billion barrels to date. The Main Body 'B' (MBB) and Western 31S (W31S) sands occur at an average depth of 6,500 ft. The zone was discovered in 1941. The field was held in reserve and only periodically produced until 1976 when it was opened up to production at capacity. The current production from the MBB and W31S reservoirs is about 11,300 BOPD and 3,500 BOPD respectively. The mechanism for primary production from for the MBB and W31S reservoirs was a combination of solution gas, partial water drive, and some contribution from gravity drainage. The primary recovery factor was estimated to be in the range of 12% of the OOIP. Since the onset of the exploitation, secondary recovery was considered essential for the MBB and W31S reservoirs. P. 227
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