A Case Study of Thief Zones in a California Waterflood

Abstract

In strongly folded reservoirs lying in tectonically active areas characterized by thrust faults, thief zones can be generated by pressure parting at surprisingly low injection pressure gradients. If thief zones parting at surprisingly low injection pressure gradients. If thief zones cannot be prevented, the best remedy, is to reduce injection pressures to below the parting pressure. Introduction In an ideal waterflood, the flood front should progress evenly through the entire oil-bearing sand body. This ideal is difficult to attain in waterfloods in thick California reservoirs. In many of these floods, water penetrates thin layers of the reservoir prematurely. penetrates thin layers of the reservoir prematurely. These layers are called "thief zones" because they draw off large quantities of water and thereby impair the economics of the waterflood operation. For the purpose of this paper, a thief zone is defined as a relatively thin layer comprising 5 percent or less of the net pay thickness and taking more than 25 percent of the injected water in a given well. The detrimental effect of thief zones and the need for corrective measures have been recognized for some time. Although this paper describes briefly a number of thief-zone control efforts in the San Miguelito field, its main objective is a diagnosis of the nature of thief zones. Thus, it is oriented primarily toward gaining an understanding of the mechanism that causes thief zones to occur, with the hope that such knowledge will help to keep these zones under control in the future. Field Description and Geology The San Miguelito field of Ventura County, Calif., is located 5 miles northwest of the city of Ventura. The field is one of several located on the Ventura anticlinal structure. The San Miguelito field structure is asymmetrical and elongated, trending northwest-southeast as shown in Fig. 1. Its anticlinal appearance is caused by compressional forces that caused folding and thrust faulting. Fig. 2 shows a north-south cross-section through the structure. The limbs of the fold have an average dip of 45 degrees to the south where there are oil-water contacts. The northern limits are bounded by a fault known as the Grubb Fault. Oil-productive sands of interest are designated as the First Grubb and the Second Grubb. These sands lie at an average depth of 6,500 and 8,000 ft, respectively. They are of Pliocene age in the Tertiary system and are of both the Lower Pico formation and the Repetto formation. The First Grubb and Second Grubb zones lie between Electric Log Markers G and M. The average thickness of these formations is about 2,100 ft, of which 50 percent, or 1,050 ft, may be considered as net effective pay. Reservoir and fluid properties are summarized in Table 1. The formations are turbidite deposits. As such, they are a sequence of shales and poorly sorted sands. Because of the manner in which the beds were laid down, they are lenticular and graded. Major shale beds, however, are correlative across the structure. The sandstone gradation usually consists of a conglomeratic sand at the bottom of the bed, with a coarse-to-fine sand transition toward the top of the bed. Interbedded shale streaks exist throughout the formations. Sandstone bed thicknesses range from less than 1 ft to 15 ft or more. Waterflood History The First Grubb waterflood was initiated in Aug. 1968. JPT P. 1385