Abstract

"Bashing" is a term that is used to describe a common observation in unconventional reservoir operations where the rate transient behavior of a well is dramatically altered (most often, dramatically deteriorated) when offset wells are drilled and stimulated. Numerous working hypotheses have been proposed in order to explain the causes of such behavior; for example, loss of fracture conductivity, or well and fracture pressure interactions. Each hypothesis implicates dramatically different optimal operational decisions ranging from the choice to restimulate the well or to alter future well spacing or density and stimulation type. This work proposes a diagnostic framework that is based on water and gas rates in order to distinguish between several plausible hypotheses. The diagnostics can be used in reservoir engineering workflows in order to refute or to reinforce beliefs about the reservoir and to improve decision-making regarding well and stimulation design choices. The utility of the proposed diagnostic model is demonstrated by application to data from field bashed wells.

In this work, we investigated two bashing mechanisms: fracture pressure interaction, and loss of fracture conductivity. Given synthetic well data that is generated from several presumed truths, we apply ensemble based history matching using each of the hypothesized models. The maximum likelihood match for each scenario is obtained and we apply model selection theory in order to quantify the relative likelihood for each hypothesis to explain the underlying true system that was originally used to generate the data. The model selection framework adequately quantifies the likelihood for each mechanism to be a cause for the bashed response.

Introduction

Well bashing

Thanks to the ever-increasing energy demand, unconventional gas resources from tight- and shale gas reservoirs have received great attention in past decade. In ultra-tight nature of shale reservoir, multistage hydraulic fracture is a common technique to maximize stimulated reservoir volume (SRV). As a result, it is easy to cause bashing (fracture hit), a term to describe a situation that created by a fracture from a wellbore communicating with the fracture of an offset, which appears decrease of original well production rates. Although this phenomenon has been observed for several years, people still feel confusing about the causes of fracture bashing. Numerous working hypotheses have been proposed in order to explain the causes of such behavior; for example loss of fracture conductivity and fracture pressure interaction (Hamed et al. 2013).

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