Capillary Pressure Data From NMR Logs And Its Implications On Field Economics

Abstract

Abstract The analogous behaviors of capillary pressure and 1/T2 decay versus saturation data provide a methodology for deriving synthetic capillary pressure information directly from NMR logs even if core data are not available. Estimations of irreducible water saturation4 from NMR log-based synthetic capillary pressure profiles have numerous benefits. One benefit is the enhanced accuracy of permeability estimations by using synthetic bulk volume irreducible in the bound water model. Availability of synthetic bulk volume irreducible data from capillary pressure modeling eliminates the requirements for determining the T2 cutoff time from core analysis data or assuming generally accepted constants for it (33 ms for clastics and 92 ms for carbonates). In addition to the bound water model, an alternative permeability profile can also be generated from Purcell's7 capillary pressure-permeability relation. Another benefit derived from a synthetic capillary pressure profile is the determination of Oil Water or Gas Water Contact (OWC, GWC) by simply converting the capillary pressure information to height above free water level. This information can also be compared with the Free Water Level (FWL) from formation test tools. The relative positions of FWL and OWC assist in inferring wettability14 and the degree of wettability magnitude for the subject reservoir. Relative permeability estimations from synthetically created capillary pressure data provide an avenue to make the first-approximation profiles of the effective permeability values to the phases contained in the pore space. Consequently, effective permeability profiles in conjunction with other petrophysical and fluid properties and the pressure information lead to the anticipated production rates and overall evaluation of the economic viability of a well(s) and field. Applications of the technique with inherent assumptions, limitations, and remedies for the limitations are reviewed and explained through a case study presented in this paper. Introduction One of the concerns in dealing with fluid flow through porous media is the non-chemical interaction between the fluids and porous media. This interaction results from capillary forces between the wetting and non-wetting fluids in highly curved pore structures. A wetting fluid preferentially fills a pore structure. To remove the wetting fluid from the pore space requires work to be performed. As an alternative, work must also be performed to force a non-wetting fluid into a pore space. The required work for either intrusion or expulsion of fluids in or from pores provides a measure of the capillary characteristics for porous media. The resistance to the intrusion or expulsion of a non-wetting phase into a pore structure varies with interfacial tension between the wetting and non-wetting phases, contact angle between the invading fluid and the pore throat and inversely with the pore diameter. Intrusion or expulsion of a non-wetting fluid into or from pores differs from each other in the amount of work they require. Non-uniform pore shapes and the difference between the contact angles of an advancing or receding liquid cause this hysteresis. Therefore, the imbibition and drainage curves of capillary pressure versus saturation never overlap. Considering these aforementioned capillary phenomenon characteristics, outlining the dependencies of capillary characteristics for porous media before comparing laboratory produced results against their theoretical approximations is imperative. Pore geometry, rock wettability, interfacial tension, saturation history and the difference in fluid densities affect the magnitude of capillary pressure. Hence, the comparison of any synthetic capillary pressure data should be made against the core data obtained from a rock with similar type of pore geometry and the proper saturation treatment of the sample.