On The Performance Of Horizontal Wells In Reservoirs Containing Discontinuous Shales
Authors: Belgrave J.D.M. , Bora R. 
Published: March 2010

To fully utilize the potential of horizontal wells, the well planning team must have a clear understanding of both vertical and lateral reservoir characteristics. In many instances, a major uncertainty is the degree to which the horizontal well productivity will be affected by the presence of discontinuous shales. Previous studies concerned with this problem have reported horizontal well productivity as a function of effective vertical permeability, which offers no direct indication of the effects of shale lateral extent, preferential orientation and vertical frequency on well performance.

With a numerical model study and an explicit stochastic shale generation algorithm, the influence of shale frequency, size and areal alignment, with respect to the horizontal well direction, on the well productivity was systematically investigated. The result shows that for any given frequency, well productivity is not monotonically related to shale size. Owing to the inclined flow path to the horizontal well, a local minimum in productivity exists as the shale barriers increase in size. The orientation of the horizontal well with respect to the alignment of the shale bodies was found to influence well productivity. Additionally, the productivity of a shorter horizontal well was found to be less affected by the shale bodies. Finally, an empirical correlation based on simulation results is presented. The observations presented in this paper provide new insight into vertical reservoir characterization from horizontal well pressure data, as well as horizontal well planning in reservoirs with a known directional bias in sedimentary structures.

Introduction

The last decade witnessed a substantial increase in the activities of horizontal wells mainly due to advances in drilling technology which improved the economics of drilling horizontal wells. A horizontal well can typically produce reservoir fluids at a rate two to six times that of a vertical well. The productivity of horizontal wells depends largely on the vertical permeability of the formation. The constituents that reduce the vertical permeability of a reservoir are impermeable flow barriers, such as shales, distributed throughout the formation. Since shales are the most common type of vertical permeability barriers, the term "shales" is used in the remainder of this paper to designate all the impermeable reservoir facies. Through this study we make an attempt to quantify the effect of discontinuous shales on the productivity of horizontal wells.

Two types of shale distributions affect fluid flow in the reservoir(1)- continuous shales and discontinuous shales. The shale barriers with known lateral extent and distribution within the formation are termed as continuous shales. Discontinuous shales are those which cannot be correlated between wells and are usually distributed randomly within the porous media. The spatial distribution and the dimension of the discontinuous shales cannot be determined accurately. In order to account for the uncertainty in their lateral extent and distribution within the reservoir, stochastic modelling techniques(2) are generally used to represent the discontinuous shale barriers in reservoir simulation models. There are two different ways to represent discontinuous shales in simulation models: explicit and implicit representation.