Directional Acoustic-Velocity Distribution in a Petroleum Reservoir

Experimental conditions, theoretical considerations, and results of adirectional acoustic-velocity survey in the plane of formation of an oilreservoir in Pennsylvania are given. More than 60 blasting-cap acoustic sourceswere discharged opposite the Bradford Third sand in 10 different wells locatedwithin a 1,200- by 1,350-ft rectangular array of 41 wells. Each source wasmonitored simultaneously by 11 different hydrophones strategically locatedwithin the 41-well array. Both acoustic sources and sensors were located at theformation elevation, about 2,000 ft below the surface, so that the in-situvelocities determined were in the plane of the sandstone formation. Well-to-well acoustic velocities were calculated and are presented in tabularand graphical form. The velocity data were further analyzed statistically toyield over-all reservoir velocity trends. Results of the study show that thereservoir is acoustically heterogeneous, with local variations in P-wavevelocity of as much as 20 percent. The over-all P-wave velocity trend for thereservoir was greater in the east-west direction than in the north-southdirection by about 700 ft/sec. The mean acoustic velocity for all wells in alldirections was 15,621 ft/ sec. The acoustic-velocity distribution is also correlated to directionalpermeability data obtained by oriented core analyses. Introduction The directional variation of geophysical properties affecting wave propagationhas long been known. A brief examination of the history and evolutionaryprocesses of sedimentary rock should leave no doubt about the nature of theformations. However, little attention has been given to directional propertiesin the evaluation and recovery of natural resources. Evaluation of thedirectional acoustic (seismic) velocity distribution in reservoirs is perhapsthe most promising means of evaluating directional reservoir properties. Themagnitude of directional variation can be predicted to a degree by consideringthe lithology and the geological features of an area. However, there are alwaysuncertainties in the study of wave propagation through the earth; numerousvariables influencing wave-propagation velocities have been determined. This research was initiated originally to aid in acoustically mapping thegrowth of hydraulically induced fractures in time and space, and to assist indelineating existing fracture systems. The reservoir investigated had been thesubject of other research programs, and considerable information was available.Using the available information, however, large directional variations inacoustic velocities over small areas would not necessarily have beenanticipated. As will be shown, the velocity variations were substantial andwere correlatable, to a degree, with other geophysical and reservoirproperties. Since the late 1940's and early 1950's, a tremendous amount of work has beendone in active and passive seismic monitoring and in the general area of wavepropagation through geological materials. Most of the work falls into one offour general categories:active seismic exploration,passive earthquakeor nuclear undergroundexplosion monitoring,active oil and gas welllogging, androck noise and slope-stability studies. Obviously, eachcategory directly or indirectly involves the directional nature of propagationvelocities. The first two categories, and usually the fourth, are associatedwith very low frequency monitoring systems, such as geophones usually operatingin the 0- to 50-hz range. Frequencies used in Category 3 are usually in the20-khz range since the waves typically are transmitted only 3 ft verticallyfrom a transmitter to a receiver.