Effects of Discontinuous Shales on Multizone Steamflood Performance in the Kern River Field
- Lee. L. Williams (Chevron Petroleum Technology Co.) | William S. Fong (Chevron Petroleum Technology Co.) | Kumar Mridul (Chevron Petroleum Technology Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- October 2001
- Document Type
- Journal Paper
- 350 - 357
- 2001. Society of Petroleum Engineers
- 5.5 Reservoir Simulation, 4.1.2 Separation and Treating, 2.4.3 Sand/Solids Control, 5.8.5 Oil Sand, Oil Shale, Bitumen, 5.1.1 Exploration, Development, Structural Geology, 1.2.3 Rock properties, 1.7.5 Well Control, 1.6 Drilling Operations, 4.3.4 Scale, 5.1.5 Geologic Modeling, 4.1.5 Processing Equipment, 5.5.8 History Matching, 5.2.1 Phase Behavior and PVT Measurements, 5.3.4 Reduction of Residual Oil Saturation, 5.4.6 Thermal Methods, 5.1 Reservoir Characterisation
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Steamflood performance of the Monte Cristo I lease in the Kern River field was analyzed with a detailed, 18-pattern, heterogeneous model incorporating all reservoir zones. Results were used to improve reservoir management, quantify the impact of discontinuous shales on oil recovery from individual zones, determine interzone gravity drainage, and identify bypassed oil zones for future infill drilling.
Results show that detailed modeling provides a more realistic description of actual performance. Furthermore, small pattern-element or single-sand models used in previous steamflood studies are inadequate for such reservoirs. Calculated oil recovery and temperature profiles compare well with field data. Discontinuous shales allow significant oil drainage to occur from the upper to the lower sands. As a result, the upper, unsteamed zones contain less reserve than expected. On the other hand, the lower zones continue to produce long after steam injection into the zone has stopped, giving high apparent recovery. The multipattern, heterogeneous model also may be used to identify bypassed zones for future infill drilling.
The Kern River is a shallow heavy-oil field located 5 miles northeast of Bakersfield, California. It is one of the largest oil fields in the United States based on oil in place and reserves. The field has been on steam injection since the mid-1960s.1-5 The Kern River field comprises several different leases, and steam injection projects were started at different times in various leases. Fig. 1 shows the Kern River field and the Monte Cristo I (MCI) lease. Steam injection was initiated in 1976 in the MCI project analyzed in this paper.
The Kern River reservoir is composed of a sequence of several alternating sand and shale members. Fig. 2 shows a schematic of the reservoir zones and a type log. The reservoir is generally processed from the bottom up.3-6 That is, steam injection is initiated in the lowest zone, and the remaining reservoir zones are sequentially processed. However, many of the shales are not continuous over the entire project area. Furthermore, shale continuity varies both areally and vertically.7 As a result, significant fluid migration can occur between sands, making reservoir management and analysis more challenging.
In the past, numerical simulation studies of Kern River and other steamflood projects have been conducted to interpret performance, evaluate recovery methods, and optimize field development.8-14 Because of computer memory, computation time, and other limitations, these studies have used confined, pattern-element models and usually have considered a single sand zone. Multiple zones were handled as layered models, with uniform (average) properties in a layer.13 Furthermore, to describe observed field behavior, such models typically required history-matching adjustments to relative permeability, transmissivity, and rock compressibility.13
Recent steamflood flow simulations have shown the importance of capturing detailed heterogeneity.15,16 In these studies, detailed geology was obtained through geostatistical methods. In addition, unconfined14 and multipattern steamfloods17 have been investigated. However, the combined effects of multipattern and heterogeneous models on steamflood performance has not been investigated before. Furthermore, the effects of discontinuous shales on multizone steamflood response has not been properly evaluated. In addition, the variation in production response of steamflood infill wells has not been investigated.
This paper evaluates the steamflood performance of the Kern River Monte Cristo I reservoir by use of a detailed, 18-pattern, heterogeneous model containing all reservoir zones. The specific objectives of this study were to: (1) develop a multipattern model for improved reservoir management, (2) determine impact of discontinuous shales on oil recovery, (3) quantify interzone gravity drainage and its effect on production response, and (4) identify bypassed zones for infill drilling and quantify variation in the infill well production response.
Reservoir Geology and Geologic Model
The Kern River field produces from several distinct zones. These sands were deposited in a braided river environment and have high permeabilities and porosities. The sandstones are typically medium- to very coarse-grained, or poorly to very poorly sorted, and they have little to no detriatal clay. The high-quality reservoir sandstones are interbedded with poor quality sandstones, siltstones, and mudstones, which may be barriers to fluid flow.
The reservoir is divided into several (as many as seven, at the MCI location) reservoir zones or sands. These individual sand bodies, which are typically 50 to 100 ft thick and are separated by competent and correlatable shale layers, are steamflooded one at a time. The reservoir dips gently at approximately 3 to 4° from northeast to southwest. Typical reservoir depth is 300 to 850 ft.
Fig. 2 shows a schematic of the sand members and a type log. As shown in Fig. 2, up to 19 separate zones have been identified at the Kern River field. However, not all zones shown in Fig. 2 exist throughout the field. The updip sands pinchout and the downdip are bounded by an oil/water contact in the lower China Grade sands. At the MCI location, only C through K2R zones exist. The Kern River field properties of low reservoir pressure, high permeability, and high oil saturations are all favorable for steamflooding.
As noted earlier, the reservoir is typically processed from the bottom up. However, because the shales are not continuous over the project area, significant fluid drainage can occur between sands. Therefore, proper modeling of these shales is necessary to capture fluid movement accurately.
Detailed geologic models were constructed for the project area shown in Fig. 3. The project area is 61 acres, and it covers about one third of the MCI lease. It consists of 22 five-spot patterns with 74 wells. Three of these wells had core data. Currently, there are 53 active wells (20 injectors and 33 producers). In addition, four temperature observation wells are located in the project area. The average pattern size is 2.6 acres, with an average injector-to-producer distance of 240 ft. The high well density at Kern River provided good well control for the detailed geologic model.
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