The Jurassic conglomerate reservoir in Xinjiang province, northwest China, is one of the largest conglomerate type reservoirs in the world. Hydraulic fracturing stimulation has become a necessary and crucial development method for the hydrocarbon reserve in the region of concern. To efficiently guide the hydraulic fracturing operation, it is first desirable to predict the fracture gradient accurately. Firstly, the fracture gradient (FG) was calculated by Matthews & Kelly's method, Eaton's method, and Andersen's method. Then, six sets of leak-off test (LOT) are analyzed. By comparing the relationship between the two, it is found that the LOT value of the conglomerate section is much larger than the calculated value, and the LOT value of the sandstone section is close to the calculated value. This paper proposed an improved method for predicting fracture gradient for Jurassic conglomerate formations in Xinjiang. The gravel diameter correction coefficient η and the LOT correction factor φ were introduced. So, the improved fracture gradient prediction method can correct the influence of gravel diameter on the fracture pressure and the difference between the fracture gradient obtained by the LOT and the calculated values. The method can efficiently aid field engineers to optimize the hydraulic fracturing design or to select more reasonable and safer drilling fluids.

1. Introduction

The Jurassic conglomerate reservoir in Xinjiang province, northwest China, is one of the largest conglomerate reservoirs in the world. Figure 1 shows the location map of the study area. The reservoir is featured by an extremely low permeability and substantial heterogeneity, while the natural fractures rarely exist. To more efficiently develop conglomerate reservoirs, hydraulic fracturing has become a necessary and crucial stimulation method in the region of concern. Fracture gradient refers to the fluid pressure beyond which the formation produces hydraulic fractures or opens the original fractures. It is one of the basic parameters required for drilling and fracturing designs.

Keywords:
reservoir geomechanics, well logging, drilling fluid chemistry, log analysis, hydraulic fracturing, fluid loss control, reservoir characterization, drilling fluid selection and formulation, drilling fluids and materials, well control
Subjects:
Pressure Management, Drilling Fluids and Materials, Hydraulic Fracturing, Reservoir Characterization, Formation Evaluation & Management, Well control, Drilling fluid selection and formulation (chemistry, properties), Reservoir geomechanics, Open hole/cased hole log analysis
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2019. Japanese Society for Rock Mechanics. Permission to distribute - International Society for Rock Mechanics and Rock Engineering
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