Carbonate Reservoir Characterization: Bridging the Gap Between Core and Seismic

Abstract Summary We demonstrate the application of rock physics as the linchpin in inverting geophysical measurements for properties that govern hydrocarbon volumes and production rates. The importance of having internally consistent rock physics models is essential for quantitative seismic inversion. This paper also highlights many of the effects that need to be incorporated into an analytical rock physics model. Some of the applications include improved reservoir characterization in carbonates, use of rock physics in 4-D analyses and interpretation, and validation with computational rock physics. We first describe the inversion process and the requirements for generating an earth model or models containing engineering and geologic rock and fluid properties. We will discuss extending the Xu-White (1995) rock physics model to carbonate rocks. We will then demonstrate pore type inversion by fitting Vp and Vs data simultaneously to the carbonate rock physics model to invert for either crack pore volume fractions or stiff pore volume fractions. Rock Physics Modeling When inverting geophysical data, the primary goal is to predict engineering and geologic properties that describe hydrocarbon volumes and the rate at which we can produce reservoirs. These properties include porosity, lithology (often indicated by Vshale), water saturation, permeability, pore type and shape, stress state, and others. A crucial step in this process is the development of internally consistent rock physics models that must be used to convert from geophysical data to these properties. The development of a carbonate rock physics model is extremely difficult because carbonate pore systems are more complex than they are in clastics. Carbonates can have a variety of pore types, such as moldic, vuggy, interparticle, intraparticle and micro pores. The complex pore system creates significant scatter in the porosity velocity relationship, as indicated in experimental results (e.g., Anselmetti and Eberli, 2001). Pore shape appears to be the dominant factor in carbonate rock physics. Moldic, intraframe, and vuggy pores tend to make the rock stronger (faster) than rocks dominated by interparticle porosity. Micro pores (e.g., microcracks) tend to be flat and make the rock weaker. To effectively characterize carbonate reservoir rocks, it is critical to develop a rock physics model capable of handling different pore types. In addition to the size and shape of pores, other factors need to be included in a physics-based rock model. Some specific additional factors are lithology and grain shapes, multiphase fluids and wetting effects, rock-fluid interactions (poro-elasticity and chemical changes to the framework), stress effects, anisotropy, heterogeneity and scale effects, and corrections for environmental effects due to logging conditions. Any rock physics model should be calibrated and validated with controlled laboratory experiments, field measurements, and computational rock physics.