Rock Characterization in Reservoirs Targeted for Horizontal Drilling

Summary

Achieving the maximum economic benefit from horizontal drilling requires thorough understanding of reservoir characteristics. The direct measurement of rock properties from oriented core is critical in horizontal-wellbore design. This paper outlines the measures and testing necessary to evaluate naturally fractured reservoirs effectively with field and laboratory technologies. Rock mechanical properties, fracture strike, and principal in-situ stress magnitudes and directions should be known before a horizontal wellbore is drilled. These data can then be used to maximize the intersection of natural fractures and to minimize the potential of borehole failure. In exploration wells, a vertical pilot hole must first be drilled. The zone of interest is cored, field tests are performed, laboratory testing is completed, and the reservoir is evaluated. With this information available, decisions can be made to optimize the borehole azimuth and well placement. We have used this approach to formation evaluation in several reservoirs where rock characterization is essential in the exploration and drilling program.

Introduction

Since 1985, the oil and gas industry has experienced an enormous increase in horizontal drilling. In preparation for drilling a horizontal well, a number of existing technologies can be used to increase the chances for a successful exploration program. Oryx Energy Co. has effectively used vertical-pilot-hole oriented coring, rock characterization, and openhole small-volume hydraulic fracturing to evaluate formations targeted for horizontal completions. These methods are particularly useful in exploration wells with little or no available data. This approach also allows timely evaluation of the reservoir before the horizontal well section is drilled. First, the well is drilled vertically (Fig. 1) to the zone(s) of interest, and microfracture tests are performed to determine the in-situ stress magnitude. Oriented cores are taken to measure the in-situ stress direction, and mechanical tests are done to determine the rock failure criteria. These data must be included in the design of the horizontal section of the wellbore a priori to achieve favorable results. The well can then be plugged back and kicked off horizontally in the optimum direction determined by the analytical program.