The Xujiahe group is comprised of a sequence of sandstone sediments deposited in the area of the present Sichuan Basin, China, between the lower Cretaceous and upper Jurassic. The sandstones are very tight and strong, and are difficult to be hydraulically fractured in the field. Therefore, there is substantial interest in reducing the hydraulic fracturing breakdown pressure. Cyclic loading has been considered as one of the methods to reduce the breakdown pressure. This paper presents the results of an initial experimental study of the effect of cyclic loading on the breakdown pressure of the Xujiahe sandstone. Two batches of specimens with different strengths were hydraulically fractured in a triaxial system. For each batch, both conventional injection tests (increasing the injection pressure monotonically until failure) and cyclic injection tests were conducted. In each cyclic injection test, the injection pressure was cycled first and then increased until the specimen was fractured. In addition, the mechanical properties of each specimen (e.g., tensile strength) were measured. Correlations of these properties and breakdown pressures were obtained. Test results show that the rock tensile strength plays a more important role in the formation breakdown than it has been expected. For the stronger batch of specimens, the average breakdown pressure for cyclic injection is 34.90 MPa, while for conventional injection it is 43.03 MPa. For the weaker batch of specimens, the average breakdown pressure for cyclic injection is 25.20 MPa, while for conventional injection it is 27.15 MPa. This indicates that the effect of cyclic loading is more significant on relatively stronger rock specimens. Based on the results, we anticipate that the in-situ formation breakdown pressure will be reduced by applying cyclic loading.

1. INTRODUCTION

The Xujiahe group is comprised of a sequence of sandstone sediments deposited in the area of the present Sichuan Basin, China, between the lower Cretaceous and upper Jurassic Epochs. The sandstones are tight gas reservoirs. Geologically, these rocks were densified by tectonic movements and are very tight and strong. As a result, it is difficult to hydraulically fracture the rocks in the field. Therefore, there is substantial interest in reducing the hydraulic fracturing breakdown pressure. In two field tests, we discovered that the formation breakdown pressure could be reduced by cycling the injection pressure. Therefore, we decided to explore the effect of cyclic loading on the breakdown pressure.

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