The input of heavy metals by automobile exhaust pollution associated with the ski basin activities is the primary concern of this paper. Stream, snowpack and lichen samples were collected and analyzed for Pb, Zn, and Cu. Some lichen samples were also analyzed for Ca, Mg, Na, K, Fe, and Mn, as well as Pb, Zn, and Cu. Snowpack samples from roadside areas demonstrated increased levels of Pb and Cu in comparison to areas up to 180 m from the road, but heavy metal levels at more remote areas were comparable to roadside levels. In all snow samples the heavy metals associated with the particulate matter was the major form of all inputs. Heavy metals in streams were also highly correlated with the particulate matter content. Heavy metal inputs to the watershed were greater than stream export, indicating an accumulation of heavy metals by the system. The forest canopy appears to be the major factor in the accumulation of heavy metals, probably due to better interception and impaction of the particulate matter by the canopy.
Eighteen horizontal wells have been completed in the 26R reservoir, Elk Hills Field, CA, through January 1995. The horizontal wells have been critical in maintaining high oil production rates from the reservoir and have significantly increased ultimate recovery. The 26R reservoir is a steeply dipping, layered, turbidite channel-sand deposit in the Miocene Monterey Formation. The high dip and low kv/kh ratio make gravity drainage the dominant production mechanism. The reservoir operating policy has been full pressure maintenance through updip gas injection since 1976. Horizontal drilling was first undertaken to develop reserves that could not be effectively recovered by conventional vertical wells in the thinning 26R oil column overlain by an expanding gas cap. Gravity drainage along the dipping bedding planes into horizontal wellbores placed just above the oil-water contact provides an effective recovery mechanism in such an environment. Over time, the horizontal wells have become the principal factor in determining the development/depletion strategy for the mature 26R reservoir. They have been successful not only in increasing oil production rates and ultimate recovery but also in controlling gas production, reducing injection requirements, and minimizing related operating costs. This paper discusses the evolution of the reservoir development strategy utilizing horizontal wells, the geologic factors dictating changes in the design of the wells, and the operating efficiencies afforded by the horizontal wells. Field data illustrating the critical importance of the horizontal development program to 26R reservoir performance is provided. Introduction The Elk Hills Field is at the south end of the San Joaquin Valley in central California. The 26R Sand is one of the most prolific of the many Stevens zone reservoirs found in the field. It is a massive complex of turbidite channel sands on the southwest flank of the 31S anticline (Fig. 1), dipping as much as 70 degrees. The oil column was initially about 1800 feet thick. The high dip angle makes gravity drainage the dominant drive mechanism. Full pressure maintenance through updip gas injection has been the operating policy since the 26R reservoir was put on sustained production in 1976. The reservoir was originally developed with conventional vertical wells on 10-acre spacing. Production rates in these wells were initially very high due to the thick pay interval, permeable reservoir sands, and high formation dip. The reservoir was undersaturated when production began, so that gas was initially produced at the solution gas-oil ratio (GOR) of 600-800 SCF/BO. Oil production peaked in 1981 at 53,000 BOPD and began declining steadily in 1983 (Fig. 2). The oil column became progressively thinner as withdrawals continued, and with corresponding growth in the gas cap free gas production steadily increased. As part of the pressure maintenance strategy, the vertical wells were produced until reaching a GOR of 12,000- 15,000 SCF/BO. Gas isolation work was then attempted, if feasible. Most of the wells were equipped with multiple packers and sleeves to accommodate this work. Controlling gas production became increasingly difficult as the oil column became thinner and the oil was overridden by the more mobile free gas. By 1988, the oil column had been drawn down to approximately 250 feet or less. Many of the wells were producing inefficiently at high GORs and no longer had enough oil productive interval left to attempt gas isolation work. The pressure maintenance strategy required that wells with no further gas isolation potential be shut in when gas production became excessive. Drilling additional vertical wells to recover the remaining reserves from the thinning 26R oil column in the presence of a high pressure gas cap was considered a marginal economic prospect. Thus, horizontal drilling was proposed as an effective means to exploit these reserves.
This paper is a field case history on the performance of the 26R Reservoir. This is a gravity drainage reservoir under pressure maintenance by crestal gas injection. The 26R Reservoir is a highly layered Stevens turbidite sandstone. The reservoir is located in the Naval Petroleum Reserve No.1 (NPR-1) in Elk Hills, Kern County, California (Figure 1). The 26R Reservoir is contained within the steeply dipping southwestern limb of the 31S Anticline. The reservoir had an initial oil column of 1,800 feet. Original oil-in-place (OOIP) was estimated at 424 million barrels.1 Pressure maintenance by crestal gas injection was initiated immediately after production began in October 1976. The total volume of gas injected is ahout 586 BCF. This exceeds one reservoir pore volume. Reservoir pressure has declined from 3,155 psia to 2,46l psia at 6,000 feet subsea. This pressure decline is believed to be due to migration of injected gas into the overlaying shale reservoirs. Under the gas injection pressure maintenance strategy, reserves are estimated to be approximately 212 million barrels. Reservoir studies have concluded that the aquifer at the base of the reservoir has been relatively inactive. Well recompletions, deepenings, and horizontal wells are used to improve oil recovery. An aggressive program of controlling gas production began in the mid 1980's by the installation of multiple packers and sleeves. As the gas-oil contact (GOC) has dropped, sand intervals have subsequently been isolated behind packers. A cased hole logging program was recently undertaken to identify possible remaining reserves in the gas cap. Introduction The Naval Petroleum Reserve (NPR-1) in Elk Hills, is located in the Southern San Joaquin Valley of Central California, about 20 miles WSW of Bakersfield and 10 miles north of Taft (Figure 1). NPR-1 contains three large anticlines, the 3lS, Northwest Stevens and 29R Structures (Figure 2). The 26R Reservoir is located within the steeply dipping southwestern limb of the 3lS Anticline. It is a Stevens turbidite sandstone reservoir in the uppermost portion of the Elk Hills Shale member of the Monterey Formation (Figure 3). The reservoir is contained within productive shales and adjoins the Western 31S and Main Body ‘B’ Reservoirs. The 26R Reservoir had an initial oil column of 1,800 feet and a net prbductive thickness of approximately 1,150 feet. Pressure maintenance by crestal gas injection was initiated immediately after production began in October 1976. However, due to suspected migration between the 26R Reservoir and the overlying shale reservoirs, the pressure has declined steadily from the initial level of 3,155 psia to the current level of 2,461 psia at 6,000 ft. subsea. Since initial production, extensive reservoir evaluations have been conducted to establish an optimum operating strategy. Crestal gas injection for pressure maintenance in the 26R Reservoir has been a highly successful strategy for a layered, steeply dipping, gravity drainage dominated sand reservoir. The reservoir and production engineering activities are presented in detail in this paper.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.