Shell Western E&P Inc. will begin a major campaign to tap oil from the almost intractable diatomite pay that underlies California's South Belridge and North Belridge oil fields.
Shell's total investment for its 1994 diatomite program, including primary waterflood, and steam drive proauction, probably will exceed $40 million, said Dave Montague, Diatomite Asset Unit (DAU) manager. That compares with diatomite outlays of about $5 million in 1993.
Shell estimates original oil in place at 2.5 3 billion bbl under tracts it acquired with purchase of Belridge Oil Co. in 1979. Only about 110 million bbl of oil have been recovered from the diatomite since the Belridge discovery in 1911.
"With primary production in the better part (of the pay), we might get 10% (recovery,),"Montague said. "With waterflooding, another 5%. The jury is still out on steam drive."
DRILLING
Shell plans to drill 70 80 diatomite wells this year, said Phiroze D. Patel, DAU technical manager.
About 40 50 will be infill wells in the waterflooded area of South Belridge, taking spacing from one well/1-1/4 acres to one well/5/8 acre.
About 10 more wells will be drilled for primary production, which involves fracturing to increase production, and 10 15 wells for expansion of steamflooding. Drilling is to begin very soon.
Diatomite wells will be drilled to 2,000 2,500 ft, producing oil of 10 34 gravity, with an average gravity of 29-30.
STRATEGY
The diatomite formation at Belridge is upper Miocene strata that were formed by diatoms, single celled plants that lived in the shallow sea that once covered the southern San Joaquin Valley.
The formation has high porosity, as much as 58%, but the pore throats are small, resulting in permeability as low as 1.5 md. That calls for fracturing.
The expanded diatomite campaign Shell is mounting is the result of what Montague described as "a confluence of events" that include successful pilot projects, many research studies, and improved field operations to help offset development costs.
A basic part of improved field operations involves teams formed in 1993, said Frank Dominguez, DAU field superintendent.
"Teams consist of our engineering staff, operations staff, including all of our gas plant, well surveillance, and area maintenance groups and well serivicing operations," Dominguez said. "We're also involving our vendors in the team effort."
Teams focused on critical business issues, looking at well optimization, failure analysis of all surface and subsurface operating equipment, and preventive maintenance.
"Basically, it was a realignment effort," Dominguez said. "We realized an 11% decrease in operating expenses. "
The stepped up diatomite program also is seen as more than a 1 year effort.
"Our vision is that we run this for 4 years," Montague said. "And then do we expand?"
Patel noted that Shell wants to spread the risk. "Others are welcome to participate in the steam drive process," he said. "In return, they may have benefits of the data and findings."
PRODUCTION
Shell produces about 20,400 b/d from the diatomite, with another 51,000 b/d from the Pleistocene Tulare zone that overlies the diatomite in the two Belridge fields.
Company, production of 71,400 b/d is up sharply from the 42,000 b/d the acreage was producing when Shell purchased Belridge for $3.65 billion, at the time the biggest corporate acquisition in U.S. history. Shell's production from the Belridge tracts peaked in 1987 at 128,000 b/d.
South Belridge field wide production in 1993 averaged about 127,000 b/d. North Belridge produced about 6,000 b/d on average in 1993.
Shell also produces about 32 MMcfd of gas from the diatomite.
It injects about 65,000 b/d of water through about 300 injection wells employed for waterflooding. Plans call for injection of 200 400 b/d/well in new waterflood wells. The waterflood covers about 50 acres.
In the steam drive project, Shell is injecting the equivalent of 200 b/d of water in each of two injection wells. The expansion win add 13 injection wells, with rates of about 150 200 b/d. The steam drive covers about 10 15 acres.
