Steam Injection Project In Heavy Oil Diatomite
Authors: Murer A.S. , McClennen K.L. , Ellison T.K. , Timmer R.S. , Larson D.C. , Wolcott K.D. , Walker T.M. , Thomsen M.A. 
Published: December 2007

A steam injection project has been conducted in diatomite containing heavy, biodegraded oil (120 API, 3000 cp.) in the South Belridge field, Kern County, California. The diatomite interval tested (San Joaquin, Etchegoin, and Belridge diatomite) 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, or OSR's, are less than 0.1). The well was then hydraulically fractured and propped. Two additional steam cycles were completed, with considerably greater oil recovery (OSR's &gt;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 greater than one year. During the steamflood, heavy oil has been 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 (&lt;0.1 OSR), but steam injection is shown to be a promising recovery technique for the heavy oil diatomite.

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 holdings in South Belridge Lost Hills, and McKittrick contain in the range of 1–2 billion barrels. These formations are marked by high porosity (40–70%) and moderate to high oil saturation that can result in very high oil concentrations amenable to such recovery techniques as steam injection. The low permeability of diatomite (generally &lt; 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 by both majors and independents. These thick diatomite strata (up to 1000') 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 steeply. Mobil's South Belridge could certainly benefit from such a synergistic implementation. Mobil's South Belridge diatomite reservoir exhibits considerable areal and vertical variation in oil properties. In the central and southeastern portions of Mobil's properties, the upper portion of the diatomite reservoir contains heavy, biodegraded oil, as is 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 has primary recovery operations for light oil in both the Opal 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. Even so, the ultimate expected recovery is small (&lt;20%, even for waterflood).

Currently, Mobil has no commercial operations in the heavy oil diatomite. Heavy and intermediate oil, at least for Mobil's South Belridge, represent a significant fraction of the total holdings. Commercial cyclic steam operations have been ongoing by Union, Chevron, and Texaco in the McKittrick field and pilot operations for cyclic and steamflood have been initiated by Cal Resources and Mobil in the South Belridge field.

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