Effects of pressure dependent leakoff and process zone stress on hydraulic fracture treatments and production.

摘要

Hydraulic fracturing continues to be the primary mechanism to produce hydrocarbons out of tight gas and other unconventional reservoirs like coals and shales. It has been studied extensively over the past few decades. However, there are factors that have been either ignored or diagnosed incorrectly leading to screenouts and/or "pressure-outs" in a stimulation treatment. Assuming that a majority of the perforations are open and there are no issues with the stimulation fluids, screenouts and "pressure-outs" during hydraulic fracture treatments can be attributed to either high pressure dependent leakoff (PDL), high process zone stress (PZS) or in some cases both. One of the main reasons that lead to "pressure-outs" is high process-zone stress (PZS). With high PZS, the chance for pressuring out is higher than screenout (i.e.one can still flush the job at lower rates provided the sand has not settled in the wellbore). The objective of this work is to help identify and present solutions to address these reservoir related issues such that screenouts can be avoided. Another objective is to show the effects of high PZS in stimulation treatments and the associated production from such zones. It has been found that zones that exhibit high PZS (greater than 0.2 psi/ft) have been economically poor producers. Thus, PZS can be used as a candidate selection tool for stimulation treatments. A Diagnostic Fracture Injection Test (DFIT) and/or a grid-oriented fully functional 3-D fracture simulator with shear decoupling are needed to identify PDL and PZS. In one case history, where high PZS was considered to be the dominant reason for pressure-out, the well was re-stimulated successfully by implementing the solutions presented in this work. In another case history, where high PDL was considered to be the main reason for screenout, there were several wells in the same project area that exhibited the same behavior resulting in screenouts. After implementing the solutions presented in this work to address high PDL all new wells were stimulated successfully without any issues. Examples are also presented from 3 shale wells to exhibit the effects of PZS. Finally, both PDL and PZS play a vital role when determining a surface area or a conductivity type stimulation treatment in shales.