Specifying Lengths of Horizontal Wells in Fractured Reservoirs

Abstract

Abstract When drilling a horizontal well in a fractured reservoir the aim is to intersect those fractures that are capable of providing a conduit for fluid flow. Key factors influencing the success of a horizontal well include the choice of horizon and the direction of drilling. However, even if these have been determined, the drilling distance required to achieve the aim remains a problem. New methods have been developed which allow an appropriate drilling distance to be constrained on a site-specific basis, where opening mode fractures are dominant. These methods utilize knowledge gained through empirical studies of the scaling properties of fracture populations. Statistical constraints on the required well length are calculated through measurement of the aperture size distribution of the fracture population, combined with observation of the history of fracture fill. Calculations of fracture permeability are made, based on the scale of data acquisition and are compared with matrix permeability. A case study of opening-mode fractures in the Austin Chalk reveals that large, potentially open, fractures are commonly clustered, the distance between clusters ranging from ~1m to ~50m. Aperture size distributions follow power laws and spacing size distributions are negative logarithmic or lognormal. The aperture size at which fractures are open to fluids is variable (0.14-11mm). Fracture permeability is dependant on the scale at which it is measured and has been determined at 7.1D (for 18m of Lower Austin Chalk core) and 286D (for 300m of Upper Austin Chalk outcrop). These calculated fracture permeabilities are several orders of magnitude larger than the typical Austin Chalk matrix permeability of 0.03-1.27mD.