Fluid Flow in Healed Tectonic Fractures

Abstract

Summary Core samples recovered during the course of the Multi-well Experiment (MWX)contained numerous mineralized vertical tectonic fractures a few millimeterswide. Permeabilities of these calcite-filled fractures were higher but of thesame order as the surrounding matrix. Increases in permeability of up toseveral-thousand-fold were obtained by treatment with permeability of up toseveral-thousand-fold were obtained by treatment with acetic acid. Acidfracturing provides a possible approach to formation stimulation. Introduction Core recovered during the course of the MWX in the Mesaverde formationexhibited numerous tectonic fractures. The majority of fractures in thepenetrated sandstones are vertical. Their abundance in the formation, particularly in the fluvial zone, is indicated by their presence in 4-in.-diameter cores. The fractures are readily identifiable because ofmineralization, which, in most cases, appears to fill the fracture space. Thepermeability of the rock matrix, however, is generally only a few microdarcies, and it is not obvious whether the mineralized fractures serve as a conduit oras a barrier to gas flow. The flow properties of mineralized fractures weretherefore included as part of a broader investigation of the petro-physicalproperties of low-permeability petro-physical properties of low-permeabilitysandstones from the MWX. Gas production from the Mesaverde formation measuredduring the course of the MWX was often one to three orders of magnitude greaterthan predicted from measurements of matrix permeabilities. It was concludedthat natural fractures contribute significantly to observed production. A modelof fracture distribution, based on MWX results and other observations, wasproposed for horizontal fluvial reservoirs. proposed for horizontal fluvialreservoirs. It was postulated that damage to natural fracture systems duringthe course of hydraulic fracturing could explain the reduction of productivityto below prefracture values. productivity to below prefracture values. Finleyand Lorenz recently reported on a detailed study of fracture distribution andtype in MWX cores. A compilation of these results shows the relative abundanceof vertical calcite-healed fractures. In some of the mineralized fractures, crystal growth from the fracture walls is only partial. Thus, in the reservoir, some fracture areas may have very high conductivity and as-yet-unknowncontinuity, which may dominate production behavior. The presence of numerousvertical mineralized fractures (VMF's), however, poses a number of questionswith respect to their contribution to reservoir behavior and response tostimulation. In this paper, we summarize an investigation of (1) flowproperties of core plugs containing VMF's relative to the neighboring matrix, (2) relative permeability behavior of VMF cores, and (3) permeabilityenhancement through refracturing and acidization of mineralized fractures. Experimental Methods Porosities, Surface Areas, and Porosities, Surface Areas, andPermeabilities. Porosities were determined Permeabilities. Porosities weredetermined gravimetrically by liquid saturation. Reported surface areas weremeasured by the single-point BET method with nitrogen as adsorbent and heliumas a carrier. Numerous variables and uncertainties arise in attempts to measurethe flow properties of low-permeability gas sands. properties oflow-permeability gas sands. Determination of in-situ stresses and satisfactorymethods of representing formation conditions in the laboratory arelong-standing problems. Permanent changes in core properties problems. Permanent changes in core properties can arise from drilling, strain-relaxationeffects, cutting of test plugs, contact with drilling filtrate, drying, andother aspects of core storage. Other considerations that enter into laboratorytest procedures are core orientation, choice of test fluid, pore pressures, transient effects, variation in pressures, transient effects, variation in meanconfining pressure in measurements for Klinkenberg plots, procedures forchanging water saturations in relative permeability measurements, and possibleleakage of fluid between the core surface and sleeve material. These factorscan contribute to obtaining results of limited significance. Thus, testing oflow-permeability gas sands involves compromises. Permeability, desorptionisotherms, and relative permeability measurements confirm that it is obviouslypreferable to use preserved cores (cores that have not been dried preservedcores (cores that have not been dried since recovery from the field) ratherthan dried cores, so that irreversible changes in clay structure and hydrationstate can be avoided. However, only a small number of the available MWX coreshad been protected from evaporation after recovery, and in many instances, itwas found that the adopted sealing procedure did not prevent drying. This, coupled with the problems of having enough preserved core samples containingfractures and then readily identifying them in their sealed state, made the useof preserved core infeasible. preserved core infeasible. JPT P. 1310