Geomechanical mechanisms involving faults and fractures for observed correlations between fluctuations in flowrates at wells in North Sea oilfields

Abstract

AbstractThe hydraulic conductivities of faults and fractures in reservoirs can be influenced by geomechanical perturbations due to production operations. It is anticipated that such dynamic permeabilities will be manifest as changes in flowrates at production and injection wells. Statistical correlations in flowrate fluctuations between wells from fields in the North Sea appear to bear out this expectation; they are characterized by high correlations over very large separation distances between wells, and appear to be stress-related and fault-related. This paper discusses possible geomechanical mechanisms that might explain orientational characteristics of correlations relative to stress state: (1) creation of new Andersonian, bimodal faults; (2) shear slip on pre-existing structural features according to slip tendencies; (3) polymodal faulting more consistent with three-dimensional strain; and (4) dilatation or compaction of aligned compliant (micro-)fractures at near-critical densities arranged in en echelon patterns. Mechanism 4 is currently preferred in that it is consistent with: (a) established theory of the nucleation of shear failure; (b) interpretations of widespread microfractures in the lithosphere from observations of shear-wave splitting; (c) (as shown in this paper) the observed frequencies of directionalities in oilfield flooding schemes, an independent empirical feature of production data.