Summary
A steam injection project was conducted in diatomite containing heavy, biodegraded oil (12°API, ?3,000 cp) in the South Belridge field, Kern County, California. The diatomite interval tested (the San Joaquin, Etchegoin, and Belridge diatomites) underlies an active steamflood in the sandstone of the Tulare formation. Initially, the test was to determine the viability of cyclic steam recovery from an unpropped, steam fractured completion in the diatomite. Four standard steam cycles were completed, with sluggish oil recovery [oil-steam ratios (OSR) were less than 0.1]. The well was then hydraulically fractured and propped. Two additional steam cycles were completed that had considerably greater oil recovery (OSR>0.2). The project was then configured for steamdrive by drilling a closely spaced producer. The new producer was initially completed with a propped hydraulic fracture and cycled once. The original cyclic producer was converted to continuous injection, and a two-well steamflood was operated for more than 1 year. During the steamflood, heavy oil was mobilized and response has been continuous. The configuration of the "pattern," with only one producer, results in poor capture efficiency. The performance of this incomplete pattern has been, as expected, poor (<0.1 OSR), but steam injection is shown to be a promising recovery technique for the heavy oil diatomite. The process is applicable to California diatomites, or any other high porosity, low permeability, shallow reservoirs that contain a significant concentration of heavy oil.
Introduction
It is estimated that the diatomite in the San Joaquin Valley of California contains as much as 10 billion barrels of oil. Mobil's former holdings in South Belridge, Lost Hills, and McKittrick, now part of Aera Energy, a joint venture between Mobil and Shell, contain on the order of 1 to 2 billion barrels. These formations are marked by high porosity (40 to 70%) and moderate to high oil saturation that can result in very high oil concentrations that are amenable to such recovery techniques as steam injection. The low permeability of diatomite (generally <1 md), however, makes any recovery technique very challenging.
The diatomaceous facies of the Monterey formation is widespread along the western and central portion of the San Joaquin Valley and is one of the reservoir intervals for commercial production from the Lost Hills, South Belridge, McKittrick, Midway-Sunset, and Buena Vista fields. In some of these fields, such as South Belridge, productive diatomite reservoirs directly underlie highly productive massive steamflood operations in sandstones of the Tulare formation. These thick diatomite strata (up to 1,000 ft) form an attractive target and, in some respects, represent the final frontier for thermal recovery operations in onshore California. Especially attractive, if thermal operations can be utilized to unlock the diatomite, is the existing steamflood infrastructure available for the diatomite, particularly as conventional operations (such as the Tulare) decline sharply. South Belridge could certainly benefit from such a synergistic implementation.
The South Belridge diatomite reservoir exhibits considerable areal and vertical variation in oil properties. In the central and southeastern portions of what were Mobil's properties, the upper portion of the diatomite reservoir contains heavy, biodegraded oil, the kind found in the overlying Tulare. Below this, the oil grades to intermediate and light. Further complicating the description is the mineralogy: the highly porous Opal A lies in the shallower depths, but has changed, due to increased temperature accompanying burial, from amorphous opaline silica to the less porous, more mechanically competent Opal CT. In South Belridge, Mobil had primary recovery operations for light oil in both the Opals A and CT, and waterflood operations in light (overlapping into the intermediate) oil in the Opal A. All wells for these operations are hydraulically fractured, a technique that opened the way in the late 1970's for accelerated development of the diatomite reservoirs in the San Joaquin Valley.1 Even so, the ultimate expected recovery is small (<20%, even for waterflood).
Currently, Aera has no commercial operations in the heavy oil diatomite. Heavy and intermediate oil, at least for the former Mobil portion of South Belridge, represent a significant fraction of the total holdings. Commercial cyclic steam operations have been ongoing by Union, Chevron,2 and Texaco in the McKittrick field and pilot operations for cyclic and steamflood have been initiated by Cal Resources and Mobil (now combined as Aera Energy) in the South Belridge field.3–9 During the late 1980's, Mobil had several isolated field trials of cyclic steam injection in wells hydraulically fractured and propped in intervals containing either heavy or intermediate oil. These tests paved the way for our first intensive pilot to determine the feasibility of thermal operations in the heavy oil diatomite at South Belridge.
Previous thermal pilots in the diatomite for heavy oil have utilized cyclic steam. The only previous pilot for steamflooding,3–9 also in the South Belridge, targeted a light oil interval. This pilot therefore represents the first cyclic steam followed by steamflooding for a heavy oil interval in the California diatomite.
An additional area for concern to be addressed in this thermal pilot was how steam injection would affect the problematic subsidence in the diatomite10,11 and potential wellbore failures.12,13
Original Purposes of the Test.
The initial purpose of the test was to determine the viability of high pressure steam injection into an unfractured interval of diatomite in the South Belridge containing heavy oil. The test had the following original objectives:quantify incremental oil production attributable to steam stimulation;better define the crude oil gravity and viscosity in the South Belridge diatomite;confirm the laboratory-based predictions of siliceous matrix dissolution and crude distillation resulting from steam injection;determine the feasibility of linkage to the natural fracture system; anddetermine the impact of steam cycling on localized formation compaction.
