Changes

Natural fractures are important factors in successfully completing a well. The probability of a vertically drilled hole intersecting fractures is considerably less than horizontal or slant holes. For example, at the U.S. Department of Energy Multiwell Experiment site in the Piceance basin of Colorado, a slant hole was drilled through lenticular gas reservoirs. The hole was then deviated to horizontal in a blanket reservoir. Fifty-two fractures were reported from 266 ft (81 m) of core taken from the slant hole part of the hole. In contrast, a nearby vertically drilled hole penetrating the same slant hole interval encountered one fracture, and, in the horizontally drilled part of the hole, 37 fractures were reported from 115 ft (35 m) of core.<ref name=Lorenzandhill_1991>Lorenz, J. C., and R. E. Hill, 1991, Subsurface fracture spacing: Comparison of inferences from slant/horizontal core and vertical core in Mesaverde reservoirs, ''in'' Rocky Mountain regional/low permeability reservoirs symposium: Society of Petroleum Engineers, p. 705-716.</ref> In a more recently drilled 14,950 ft (4557 m)-deep well in the Green River basin of Wyoming, more than 400 open fractures were detected on a Formation MicroImager log from a 1750 ft (533 m)-long horizontally drilled leg in the Upper Cretaceous Frontier Formation.<ref name=Krystinikandlorenz_2000>Krystinik, L. F., and J. C. Lorenz, 2000, Do you want the good news or the bad news? . . . New perspectives on basin-centered gas from horizontal drilling, Frontier Formation, SW Wyoming, ''in'' 2000 basin-centered gas symposium: Rocky Mountain Association of Geologists, unpaginated.</ref> In the same well, approximately 76 natural fractures were recorded from a 78.2 ft (23.8 m)-long core taken from the same horizontal leg.<ref name=Lorenzandmroz_1999>Lorenz, J. C., and T. H. Mroz, 1999, Natural fracturing in horizontal core near a fault zone: The Rock Island Unit 4-H well, Green River basin, Wyoming: Integrating geoscience and engineering data to characterize and exploit tight gas sand sweet spots: Gas Research Institute, GRI 99/0068, 81 p.</ref> From these two examples, the probability of encountering fractures in slant or horizontal wells vs. vertically drilled wells is well documented. The cost of drilling nonvertical wells, however, is considerably greater than the cost of drilling vertical wells.

Natural fractures are important factors in successfully completing a well. The probability of a vertically drilled hole intersecting fractures is considerably less than horizontal or slant holes. For example, at the U.S. Department of Energy Multiwell Experiment site in the Piceance basin of Colorado, a slant hole was drilled through lenticular gas reservoirs. The hole was then deviated to horizontal in a blanket reservoir. Fifty-two fractures were reported from 266 ft (81 m) of core taken from the slant hole part of the hole. In contrast, a nearby vertically drilled hole penetrating the same slant hole interval encountered one fracture, and, in the horizontally drilled part of the hole, 37 fractures were reported from 115 ft (35 m) of core.<ref name=Lorenzandhill_1991>Lorenz, J. C., and R. E. Hill, 1991, Subsurface fracture spacing: Comparison of inferences from slant/horizontal core and vertical core in Mesaverde reservoirs, ''in'' Rocky Mountain regional/low permeability reservoirs symposium: Society of Petroleum Engineers, p. 705-716.</ref> In a more recently drilled 14,950 ft (4557 m)-deep well in the Green River basin of Wyoming, more than 400 open fractures were detected on a Formation MicroImager log from a 1750 ft (533 m)-long horizontally drilled leg in the Upper Cretaceous Frontier Formation.<ref name=Krystinikandlorenz_2000>Krystinik, L. F., and J. C. Lorenz, 2000, Do you want the good news or the bad news? . . . New perspectives on basin-centered gas from horizontal drilling, Frontier Formation, SW Wyoming, ''in'' 2000 basin-centered gas symposium: Rocky Mountain Association of Geologists, unpaginated.</ref> In the same well, approximately 76 natural fractures were recorded from a 78.2 ft (23.8 m)-long core taken from the same horizontal leg.<ref name=Lorenzandmroz_1999>Lorenz, J. C., and T. H. Mroz, 1999, Natural fracturing in horizontal core near a fault zone: The Rock Island Unit 4-H well, Green River basin, Wyoming: Integrating geoscience and engineering data to characterize and exploit tight gas sand sweet spots: Gas Research Institute, GRI 99/0068, 81 p.</ref> From these two examples, the probability of encountering fractures in slant or horizontal wells vs. vertically drilled wells is well documented. The cost of drilling nonvertical wells, however, is considerably greater than the cost of drilling vertical wells.

Reservoir damage is another important aspect of formation evaluation. Spencer<ref name=Spencer_1985>Spencer, C. W., 1985, Geologic aspects of tight gas reservoirs in the Rocky Mountain region: Journal of Petroleum Geology, p. 1308-1314.</ref> listed several different types of reservoir damage, including (1) movement of secondary clays causing plugging of pore throats, (2) swelling of smectitic clays, (3) increasing water saturation with consequent reduction of relative permeability to gas, (4) fracturing gel compounds left in the reservoir, and (5) chemical additives causing precipitation of minerals and compounds during acidizing and hydraulic fracturing. The potential problem of swelling clays, in most cases, is minor, because most BCGAs occur in sequences where the level of thermal maturation is sufficiently high to convert swelling clays into nonswelling clays.

Reservoir damage is another important aspect of formation evaluation. Spencer<ref name=Spencer_1985>Spencer, C. W., 1985, Geologic aspects of tight gas reservoirs in the Rocky Mountain region: Journal of Petroleum Geology, p. 1308-1314.</ref> listed several different types of reservoir damage, including (1) movement of secondary clays causing plugging of pore throats, (2) swelling of smectitic clays, (3) increasing water saturation with consequent reduction of relative permeability to gas, (4) fracturing gel compounds left in the reservoir, and (5) chemical additives causing precipitation of minerals and compounds during acidizing and hydraulic fracturing. The potential problem of swelling clays, in most cases, is minor, because most BCGAs occur in sequences where the level of thermal [[maturation]] is sufficiently high to convert swelling clays into nonswelling clays.

==Exploration strategy==

==Exploration strategy==

The objective of any hydrocarbon exploration program is to progress from coarse, loosely defined ideas to refined, drillable locations. In the case of BCGAs, exploration strategies are no different and may be viewed as a four-step process that includes (1) reconnaissance, (2) confirmation, (3) delineation, and (4) sweet spot identification. The exploration phases are mostly applicable to direct BCGAs; as a consequence of the relatively new classification of BCGAs into direct and indirect types, strategies for indirect BCGAs have not been formulated, although it is obvious that source rock considerations, level of thermal maturation, and temporal relationships among hydrocarbon generation, expulsion, migration, and trap formation are very important considerations in the exploration for indirect BCGAs.

The objective of any hydrocarbon exploration program is to progress from coarse, loosely defined ideas to refined, drillable locations. In the case of BCGAs, exploration strategies are no different and may be viewed as a four-step process that includes (1) reconnaissance, (2) confirmation, (3) delineation, and (4) sweet spot identification. The exploration phases are mostly applicable to direct BCGAs; as a consequence of the relatively new classification of BCGAs into direct and indirect types, strategies for indirect BCGAs have not been formulated, although it is obvious that source rock considerations, level of [[thermal maturation]], and temporal relationships among hydrocarbon generation, expulsion, migration, and trap formation are very important considerations in the exploration for indirect BCGAs.

===Reconnaissance phase===

===Reconnaissance phase===