Pressure Transient Analyses of Short and Long Duration Well Tests in Unconventional Reservoirs

Horizontal wells are used in unconventional oil and gas reservoirs to increase production by creating large drainage surface areas and contact volumes. Production is further improved by applying hydraulic fracture stimulation in horizontal wells. Hydraulic fracturing increases well productivity via the large drainage surface of the fracture and by rejuvenating existing natural fractures as well as creating new fractures in the vicinity of the wellbore. The affected reservoir volume is known as the stimulated reservoir volume (SRV) which includes a complex flow network that creates different flow regimes. We will present several short and long pressure transient tests conducted in vertical and horizontal wells, to determine critical formation properties of the low-permeability, dual-porosity Middle Bakken and Three Forks reservoirs. Pressure transient test data were obtained via permanent downhole pressure gauges. The bilinear and linear flow regimes of the pressure buildup tests are the focus of the analyses. For this, we have presented an analytical solution using numerical inverse Laplace transform as well as closed-form approximate solutions. Flow rate transient analysis of long-duration production data were also conducted to compare with the results of the pressure transient analyses. All tests indicated that the field measured permeability is several orders of magnitude greater than permeability measured on core plugs. This indicates the presence of a network of interconnected fractures and microfractures in the stimulated near-well regions without which no significant production would result. The details of the well tests and analyses will be presented for engineering applications. Introduction In this paper we present several pressure buildup tests for horizontal and vertical wells in a North Dakota Bakken unconventional reservoir. The Bakken formation in the Williston Basin has been rapidly developing since 1980s; however, the recent advancements in horizontal well completion and stimulation technology play a key role in the success of today’s Bakken production. Pressure buildup tests, mini-DSTs and mini-frac tests are valuable sources of information for determining key reservoir properties, such as permeability. The Bakken formation is comprised of an upper and a lower organic-rich black shale and a middle silty dolostone or dolomitic siltstone, and sandstone member. The Bakken is overlain by the Lodgepole formation which consists of a dense, dark gray to brownish gray limestone and a gray calcareous shale and underlain by the Three Forks formation which is composed of thinly interbedded greenish gray and reddish brown shale, light brown to yellow gray dolostone, gray to brown siltstone, quartzite sandstone and minor occurrences of anhydrite (Kume, 1963). Historically, horizontal wells in the Bakken were drilled in the Upper Bakken formation while the recent developments have changed the focus to the Middle Bakken and Three Forks formations. Historically, when a reservoir interval has been productive it has been attributed to natural fractures (Murray, 1968). Natural fractures include regional fractures, local stress fractures and local micro-fractures resulting from pore fluid over-pressuring. Although fractures are typically determined by examining cores, image logs, and structural curvature, pressure transient tests could provide additional insight in deciphering the presence of fractures and their flow contribution.