Completion Interval Selection in the Lost Hills Field, Southeast Flank, Kern County, California

Abstract

Summary When the Antelope Shale/Reef Ridge reservoir of the Lost Hills field was developed, selection of the completion interval relied heavily on the subjective mud (or driller's) log. A qualitative analysis of the openhole well logs was developed that assists in the selection of the most productive zones in this massive fractured shale reservoir. Also productive zones in this massive fractured shale reservoir. Also briefly discussed are stimulation techniques and pressure transient testing problems, and results obtained in this layered reservoir. Introduction During early 1980, Chevron U.S.A. Inc. began widespread development of the Antelope Shale/Reef Ridge reservoir. Through 1982, 91 wells had been drilled by Getty Oil Co. and 157 wells had been drilled field-wide. resulting in the production of 8,000 B/D [1272 m3/d] of oil and 50 million cu ft/D [1.42 × 106 m3/d] of natural gas. The new development has resulted in the largest oil and gas discovery for onshore California since the mid-1970's. This development is along the southwest flank and down the nose of the Lost Hills anticline, which has produced from various formations for more than 70 years. produced from various formations for more than 70 years. The wells require stimulation to produce at economic rates, thus, this area was felt to be virtually nonproductive until the introduction of large-size hydraulic fracturing. Along with the technological advances in stimulation came an analysis technique for selecting the most likely productive zones. productive zones. Geology The Lost Hills field is a narrow, northwest-trending anticline. The Antelope Shale/Reef Ridge reservoir lies on the southwest flank and southeastern axial plunge of the anticline. Producing zones are within the Miocene-age Monterey formation and the overlying Reef Ridge shale. The main zones under the Getty Oil Co. properties are within the upper Antelope Shale member of the Monterey formation and lower Reef Ridge shale. These zones are shown by the type log in Fig. 1. The structural growth of the anticline has created a number of stratigraphic traps where siliceous mudstones and shales lense out updip into impermeable porcellanites. The organic-rich shale and stone stringers act both as source beds and as reservoirs for hydrocarbons. Clays in the mudstones and shales have inhibited their maturing and, thus, have sustained sufficient primary porosity and permeability to accumulate gas and high-gravity (40 degrees API [0.83-g/cm3]) oil. The impermeable cap and the hydrocarbon maturation within the lenses have been controlled by depth of burial. As a result, productive zones are encountered at approximately the same subsea depth throughout the pool. Overpressuring in the most productive zones has expanded the tectonically induced natural fractures in the brittle mudstones and shales. However, limited core data contradict the theory that much of the shale and mudstone sequence has been fractured naturally. Unfractured lenses probably do not communicate with adjacent lenses because probably do not communicate with adjacent lenses because of the low matrix permeabilities, and they require a complex completion interval selection, as well as hydraulic fracture stimulation, for optimum production. Succession of Completion-Interval Selection Techniques As the development of the fractured shale reservoir expanded, it was apparent that an effective technique of completion-interval selection would be required to ensure the economic success of the wells. Because of the soft composition and low competency of the formation(s) being completed, it was believed that a severe fines migration and embedment problem would occur, leaving the sandpack ineffective over time. Fracture conductivity testing performed by an independent research group showed that performed by an independent research group showed that the sandpack retained only 10 to 17% of its original conductivity after being placed under in-situ conditions and flow tested. Because embedment was discounted as a major problem, the fracture plugging was caused by fines migration. Thus, the location of the limited number of perforations used would become crucial for long-term perforations used would become crucial for long-term productivity. As the field was expanded down the nose of productivity. As the field was expanded down the nose of the anticline, the gross productive interval increased while the actual net productive interval decreased, placing greater emphasis on an effective completion-interval selection technique. Six distinct methods were used that evolved into the current technique, which will be discussed in detail in the following section. Each of these methods is outlined briefly here to show the methodology used in the development of this unique analysis. JPT P. 1058