Explosives Research To Improve Flow Through Low-Permeability Rock

Abstract

Small surface tests were conducted to determine the feasibility of using a nitroglycerine-base explosive for creating rock fractures. Prior to underground testing, surface and near-surface tests with liquid explosives showed that explosions in sheet-like layers simulating underground fractures would propagate through the layers. Successful surface tests were conducted using layers of explosive placed between glass plates and explosive-saturated sand confined in small-diameter metal tubes. Tests also showed that explosions propagate through the pores of Berea sandstone saturated with a liquid explosive. Encouraging result, were produced by a shallow test in which extensive fracturing was effected with 5 3/4 qt of nitroglycerin (NGI) detonated in limestone, and with a 50-qt charge of NGI displaced into a permeable zone at a depth of 42 ft in Green River oil shale. Cores, caliper logs, down-hole camera surveys, elevation in measurements and air-flow tests showed the effects of the latter explosion. Fracture improvement ratios determined on eight test holes ranged from 2.3 to 19.1 and averaged 8.0.

Introduction

Development of numerous petroleum-bearing formations in the United States has been retarded by lack of permeability in the rock. Although considerable effort has been expended by the petroleum industry and by government agencies to stimulate low-permeability reservoirs, not all tight sands (11 to 10 md) have responded to newly developed oil and gas recovery techniques. Results from hydraulic fracturing, fluid flooding and application of heat are commendable however, more research is needed to improve permeability in tight sands and oil shale. Present methods for improving permeability consist primarily of hydraulic fracturing, acidizing and, to a lesser extent, nitroshooting in the wellbore.

This research effort was initiated in 1963 as part of the petroleum engineering research program of the U. S. Bureau of Mines. It is supported financially by the American Gas Association (AGA) and by federal agencies, and it is part of a program suggested by the Independent Petroleum Association of America (IPAA). The research is to develop a technique to improve ultimate recovery from low-permeability oil and gas reservoirs and to improve performance of underground natural gas-storage reservoirs. The work involves an explosive method to increase recoveries from previously fractured zones. Introducing and detonating a liquid chemical explosive in these wells is intended to improve permeability by creating additional fractures and by enlarging existing fractures. This is one of two known research efforts on sheet-like layer explosions. Little information is available on this subject; however, some related work has been conducted by a few individuals and oilfield service companies. Earlier work resulted in moderate successes, near failures, numerous premature detonations and destroyed wells. One account is of a 5,000-qt NGI shot displaced into the formation and detonated in a well in the Turner Valley field during Feb., 1946. No increase in oil flow resulted.

Brewer indicated that the Tar Springs, Jackson and Benoist formations in the Illinois basin responded when voids in these low-permeability formations were filled with explosive and detonated. Further, the Cleveland and Red Fork sands in Oklahoma were reported to have responded to NGI shots in the formation. Included in U.S. patents relating to explosive fracturing are Zandmer, Brandon, Hanson and Hinson.

Results of the Stratablast process were reported at a meeting of the American Petroleum Institute in April, 1965. The multiple component systems used were generally hypergolic fluids that explode when combined in the formation. A service company recently reported a new blasting service for the petroleum and mining industries. The all-liquid system uses rocket-type fuels. The explosive is pumped into the formation and followed by a nonreactive liquid spacer; a liquid igniter penetrates through the spacer and sets off the explosion. Another system" employs a heavy slurry of metallized ammonium nitrate. The viscous slurry is placed in the well opposite the formation to be fractured and is detonated. Moderate volumes of the explosive designated as MS-80 are reported to produce temperatures up to 6,000F accompanied by extended fractures.

JPT

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