New velocity data in addition to literature data derived from sonic log, seismic, and laboratory measurements are analyzed for elastic silicate rocks. These data demonstrate simple systematic relationships between compressional and shear wave velocities. For water-saturated elastic silicate rocks, shear wave velocity is approximately linearly related to compressional wave velocity and the compressional-to-shear velocity ratio decreases with increasing compressional velocity. Laboratory data for dry sandstones indicate a nearly constant compressional-to-shear velocity ratio with rigidity approximately equal to bulk modulus. Ideal models for regular packings of spheres and cracked solids exhibit behavior similar to the observed watersaturated and dry trends. For dry rigidity equal to dry bulk modulus, Gassmann' s equations predict velocities in close agreement with data from the water-saturated rock.
An integrated workflow to estimate the hydrocarbon-in-place and recovery factor is applied in the Bakken-Three Forks petroleum system. Evaluating factors that control the generation and storage of hydrocarbon, such as the total organic carbon, maturity of shale, thickness, porosity, and permeability is a challenge in any shale play study. In addition, the hybrid nature of the Bakken petroleum system, where the source and reservoir rock are present within a short depth interval, adds complexity to the production interpretation and outlook of the play. One complexity is the contribution from Upper and Lower Bakken organic-rich shales to the production of horizontal wells completed in the Middle Bakken low-permeability laminated sandstone/siltstone and Upper Three Forks sandy/silty dolostone. We have performed geologic and petrophysical studies and calculate and map the hydrocarbon pore volume. For fluid characterization, we use three models to accurately cover a range of American Petroleum Institute gravity and gas/oil ratio. We evaluate the contribution of Upper and Lower Bakken to production by constructing simulation models and used that knowledge to estimate the recovery factor of the horizontal wells. Production depletes the Middle Bakken, creating a pressure difference between the Middle Bakken and the Upper/Lower Bakken, which in turn depletes the Upper/Lower Bakken. Vertical permeability controls production from the Upper and Lower Bakken, and higher vertical permeability increases the contribution of the two shale members. An understanding of the maturity and trap mechanism can help to explain the water-saturation distribution, and understanding these factors is crucial to any future development of the play.
Summary
Overpressure is a common feature among productive unconventional shale reservoirs, such as the Bone Spring (BSPG) and Wolfcamp (WFMP) Formations of the Delaware Basin (DB) of west Texas and southeastern New Mexico, and is thought to be a strong driver of well productivity. Compared with conventional reservoirs and shales in normal pressured conditions, the effects of overpressure on the mechanical properties of shales is not well understood. Here we present an analysis of overpressure in clay-bearing siliciclastic facies of the BSPG and WFMP Formations of the DB and implications for mechanical properties of the reservoir. Estimation of the effects of overpressure on mechanical properties of unconventional shale reservoirs is determined through use of the sonic overpressure indicator (SOPI).
The method requires log model results that accurately characterize variations in lithology and porosity for the formations of interest. The SOPI (ΔT/ΔTN)2, where ΔT is the measured compressional sonic transit time, and ΔTN is the forward-modeled result for normally pressured conditions, can be used with elastic moduli and their interrelationships to compare estimates of mechanical properties including Poisson’s ratio ν, the Biot or effective stress coefficient α, and Young’s modulus E, in normal and overpressured conditions. Results presented here are broadly applicable to overpressured unconventional reservoirs that contain significant clay volume (>0.1 v/v) and exhibit low porosity (<0.08 v/v), comparable to that of siliciclastic-rich facies of the WFMP Formation.
To account for increased VP/VS ratio, we regard overpressurization of shaly facies as an irreversible thermodynamic process that transforms a normally pressured siliciclastic system. At stress below the yield point, which is taken as the limit of normal pressure, the system responds elastically to stress; beyond this point, during overpressurization, the system responds as an elastic/plastic medium with strain hardening.
We regard elastic moduli as descriptive of mechanical energy stored in this system. This perspective enables Poisson’s ratio for the overpressured system νOP to be computed from an estimate of the normally pressured system νN using (ΔT/ΔTN)2. Overpressure also results in a limited increase of the Biot or effective stress coefficient α. Moreover, recognition that overpressure results in a decrease of Young’s modulus, that is, EOP/EN < 1, provides a means of estimating the amount of strain energy stored by the formation due to overpressurization.
We believe that when exposed to lower pressures by wellbore construction, this strain energy stored in overpressured unconventional reservoirs drives creep, which affects interpretations made using geomechanical models. We have developed and tested computational models based on biaxial or plane strain for vertical wells and uniaxial strain for horizontal wells that describe how creep likely affects estimation of minimum horizontal stress Shmin and pore pressure from instantaneous shut-in-pressure (ISIP) measurements. Thus, for overpressured unconventional reservoirs, ISIP determinations differ from tectonic Shmin by an amount related to ν and EOP/EN.
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