Using Elemental Data for Accurate Wellbore Placement and Geosteering in Unconventional Reservoirs: Examples From the Appalachian Basin

Abstract

As the pursuit of oil and gas in unconventional reservoirs grows, it is increasingly evident that horizontal wellbore placement, or targeting, plays a first-order role in the production capability of a well. Indeed, the percentage of a wellbore “in target” is a common metric used when evaluating the causes for good or poor production from any particular well. The most common process used for geosteering a horizontal wellbore into a chosen target is the correlation of logging-while-drilling (LWD) total gamma ray (GR) to a vertical pilot-hole GR log. However limitations inherent to this procedure can reduce the ability to effectively use LWD GR data. These limitations can include short GR counting intervals vs rate of penetration (ROP), GR detector sizes, and data transmission. Geologic factors such as low GR contrast from bed to bed, and repetitive GR trends, especially in areas where faulting with significant throw (defined as ≥ to thickness of the target) can further complicate the correlation of LWD GR data back to the pilot hole. In effort to more accurately geosteer wells we have employed the use of elemental data derived from energy-dispersive X-ray fluorescence (ED-XRF). Elemental data is acquired on vertical pilot holes at the heal and, where possible, near the toe of proposed laterals, at one foot intervals on core and five foot intervals on cuttings. This data is used to build a chemostratigraphic profile and zonation of the section. Chemostratigraphic zones are defined as having multiple elements (where possible) which illustrate distinct changes in chemical profiles from one zone to another. These zones must be correlative over reasonable distances (at a minimum the length of the horizontal wellbore) and must be readily identifiable in cuttings. Using these criteria chemostratigraphic zonations have been constructed in the Marcellus Shale, Lower Huron Shale, and Newman (Big Lime) Limestone. Well site ED-XRF data was used in conjunction with LWD GR to geosteer a ~25′ thick porosity zone which resides at the base of a ~400′ thick non-porous/non reservoir carbonate section of the Newman Limestone and immediately underlain by the siltstones and shales of the Borden Shale. Well site XRF data was successfully used to identify cave-ins that were mistakenly identified as the Borden Shale, determine the position of the wellbore in zones of non-descript GR signature, and determine the lateral extent of the reservoir interval.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015