When Can a Fracture Be Initiated Through a Circular Wellbore in an Oil Sand Reservoir?

Abstract

Abstract Oil sands and other poorly consolidated sandstones (PCS) behave differently under different loading procedures, and retain memory of their load histories in the three-dimensional array of contacts and forces. These factors complicate predictions of the response of PCS to external loadings. Prediction of hydraulic fracture initiation pressure in a PCS is difficult, as fracture initiation depends on in situ stress state, material constitutive behavior, and ultimately may be controlled by stress path and history. In this paper, load paths corresponding to certain field activities and laboratory tests during pumping and injection are analyzed to evaluate cases that arise when hydraulic fracture initiation is attempted in a PCS reservoir. Fracture initiation pressures following different loading paths are presented by applying elastic and elastoplastic models. Hydraulic fracturing tests conducted in oil sands in the Cold Lake region in Alberta are used as examples. The bottomhole pressure records are used to compare with our analytical results. The conclusion is that, for the same in situ stress, wellbore hydraulic fracturing pressure records in a PCS may respond totally different when different stress paths have been followed. This load history of a wellbore environment depends on drilling and pressurization history. Conventional elastic equations for predicting fracture initiation pressures are not appropriate; one must in general interpret wellbore pressure records for hydraulic fracture in PCS using elastoplastic models, incorporating material properties and loading history. Introduction Hydraulic fracturing (HF) is used in oil sands and other PCS to estimate stresses; to decrease the near wellbore pressure drop by introducing propping agents; to introduce acid, emulsifiers, or other agents to increase oil production; or to inject large quantities of steam and hot water for thermal enhanced oil recovery (EOR) operations. Relative to other EOR adjunct technologies, HF is a technologically and economically mature approach. However, the particular behaviour of PCS in many circumstances generates confusion for drilling engineers and researchers addressing issues of HF processes. Indeed, HF initiation and propagation are not yet clearly understood in poorly cemented granular materials of low cohesion. P. 647^