The paper was presented at the SPE/DOE Unconventional Gas Recovery Symposium of the Society of Petroleum Engineers held in Pittsburgh, PA. May 16–18, 1982. The material is subject to correction PA. May 16–18, 1982. The material is subject to correction by the author. Permission to copy is restricted to an abstract of not more than 300 words Write: 6200 N. Central Expwy., Dallas, TX 75206.

Abstract

Two research wells were drilled and completed in a known, yet heretofore unproductive tight sand gas reservoir in the Green River Basin. Although previous wells were drilled in the area and previous wells were drilled in the area and hydraulically fractured, there has been no commercial production. The consideration of advanced methods production. The consideration of advanced methods of stimulation design aid changing gas prices caused recent renewed interest. A systematic approach led to successful fractures of one zone in each of the two wells. Initial production rose from 107 to 4150 MCFD in one well and from 35 to 1450 MCFD in the second well after stimulation treatments.

Introduction

The Pinedale gas field is a large tight sand reservoir located in Sublette County, Wyoming (see Figure 1). The magnitude of this resource is indicated by a National Gas Survey Advisory Committee report of 1977 which lists in-place gas reserves of 37 trillion cubic feet. Eleven wells were drilled into the Fort Union formation of this field between 1939 and 1963. Six wells were subjected to conventional hydraulic fracturing. Three of these six wells later underwent massive hydraulic fracturing treatments during the period 1974 to 1976; however, these treatments failed to significantly increase the long-term production potential and there has never been commercial production from this field. Based on advances in the technology of tight sand production, Mountain Fuel Supply Company, with the production, Mountain Fuel Supply Company, with the technical assistance of Terra Tek Incorporated, initiated an extensive research and development program in the area. The program involved a critical review of all prior data, drilling of two 12,000-foot wells, flow testing of existing wells, extensive logging of the new wells, theoretical fracture design studies, and prestimulation and poststimulation flow testing of the new wells. The fracture design work included a critical investigation of in-situ stress and fracture orientation and propagation. The laboratory work included special core testing with candidate fracturing fluids and proppants. This paper presents details of the study and the associated laboratory and field results.

Funding assistance was provided by the United States Department of Energy and the Gas Research Institute.

Background

The Pinedale gas reservoir is located on the west of an anticline with a northwest-southeast trend. It is 30 to 40 miles long and approximately 5 miles wide, with approximately 2,000 feet of vertical relief from the lowest to the highest structural elevation. The anticline was formed by compressional stresses resulting from the Wind River Uplift on the east and the Wyoming Overthrust on the west during the Laramide Orogeny (appx. 100 million years B.C.). Gas-bearing sandstone units are encountered at approximately 8,000 feet in depth and continue, interbedded with shale, downward to an undefined depth, but at least as deep as the total depth (19,300 feet) of the El Paso Natural Gas Company's Wagon Wheel well. Evaluation of the data obtained from that well indicates, however, that a significant portion of the presently known gas in the field is contained in the formation interval from 9,000 to 11,700 feet. The individual sandstone units range in thickness from a fraction of an inch to several tens of feet.

Log information from other Pinedale wells and outcrop studies in the Green River Basin indicates that the Fort Union sands are lenticular with the total gross sand thickness from 500 to 700 feet. Core data measured from wells indicate porosities of 8 to 10 percent with 50 to 59 percent water saturation. El Paso Natural Gas Company investigations of the Pinedale reservoir characteristics, for samples with 8.8 percent porosity and 50.6 percent water saturation, showed permeability to gas of less than 0.001 millidarcy.

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