The objective of this paper is to outline the methodology proposed to determine the in-situ stress field and geomechanical properties of the Bakken Formation in Williston Basin, North Dakota, USA to increase the success rate of horizontal drilling and hydraulic fracturing so as to improve the recovery factor of this unconventional crude oil resource from the current 1% to a higher level. The success of horizontal drilling and hydraulic fracturing depends on knowing local in-situ stress and geomechanical properties of the rocks. We propose a proactive approach to determine the in-situ stress and related geomechanical properties of the Bakken Formation in representative areas through integrated analysis of field and well data, core sample and lab experiments. We plan to use Kaiser Effect technique to estimate in-situ stresses. CDISK method will be used to determined fracture toughness. Geomechanical properties will be measured following ISRM suggested methods. By integrating lab testing, core observation, numerical simulation, well log and seismic image, Intelligent Geomechanical Logging and Imaging methods are proposed to estimate geomechanical properties in locations where no results of lab testing and core observation are available.
The Williston Basin is a roughly oval-shaped structural down-warp with a surface area between 120,000 and 240,000 square miles. The basin underlies most of North Dakota, western Montana, northwestern South Dakota, southeastern Saskatchewan and a small section of southwestern Manitoba. All sedimentary systems from Cambrian through Quaternary are presented in the basin, with a rock column more than 15,000 ft thick in the deepest section near Williston, North Dakota (Fig. 1). The basin became a major oil province in the 1950s when large oil fields were discovered in North Dakota.
(Figure in full paper)
The Bakken Formation is a thin (maximum thickness 145 ft), naturally fractured Upper Devonian- Lower Mississippian sedimentary unit. It can be divided into three intervals: the upper shale, the lithologically variable middle member, and the lower shale. The upper and lower shales have rich organic content, and are the source rocks for oil and gas in the Bakken Formation. In North Dakota, the middle member is mainly gray interbedded siltstones and sandstones with a maximum thickness of 85 ft occurring at depths of approximately 9,500 to 10,000 ft (Heck et al., 2002).
Although the Bakken Formation is very thin compared to other oil producing horizons, it has recently attracted much attention because its extremely high carbon content places it among the richest hydrocarbon source rocks in the world. Estimates of original oil in place (OOIP) range from 200 to more than 400 billion barrels (Price, 2000). For comparison, excluding these Bakken Formation reserves, so far the total US discovered OOIP is less than 600 billion barrels, of which only less than 200 billion barrels has been produced. With the growth rate of demand outpacing that of new reserves on oil and gas, the importance of these unconventional reserves in the Bakken Formation becomes increasingly important.
