Travis Peak Core Permeability and Porosity Relationships at Reservoir Stress

Abstract

Summary. Relationships of permeability to porosity are shown from analysesof more than 2,100 core plugs from nine wells in the Travis Peak, alow-permeability, tight-gas sandstone formation in northeast Texas. Effects ofreservoir vs. ambient stress are shown for permeability, porosity, and the Klinkenberg factor. The relationship, of brine porosity, and the Klinkenbergfactor. The relationship, of brine permeability to gas permeability is alsoshown. permeability to gas permeability is also shown. Introduction In a cooperative effort, the Gas Research Inst. (GRI) and various industrypartners collected core and log data from nine wells to form an extensive database from which important correlations of porosity and permeability at bothambient and net overburden porosity and permeability at both ambient and netoverburden pressures have been developed for the Travis Peak formation. Routinepressures have been developed for the Travis Peak formation. Routine andspecial analyses were performed on more than 2,100 core plugs taken from 2,093ft of core from these wells. Fig. 1 shows the locations of the nine wells; thetwo outlined areas show where the major activity was focused. Travis Peak is asand-rich Lower Cretaceous formation with environments that include afluvial/deltaic section of low-energy flood-plain mudstones and siltstones, high-energy crevasse splays and main channel sandstones, and amarine-influenced environment with tidal flat mudstones and channel sands. Coring intervals were selected to provide data representative of all thesedifferent depositional environments. Table 1 lists the cored intervals anddepths for each well. A major factor in developing representative core analysisresults is the importance of simulating in-situ reservoir conditions for theporosity and permeability measurements. The effect of reservoir porosity andpermeability measurements. The effect of reservoir (called net overburden here)stress on permeability and porosity is particularly important forlow-permeability rocks. This has been the subject of many studies but, in most, only a few cores were involved from any particular formation or rock type. Mostof the routine analyses in this study include measurements made at both ambientand net overburden stress. As a result, these data constitute the mostextensive data base of this kind for a specific low-permeability formation. Thecore analyses were performed by four different service laboratories, and insome cases companion samples were tested by two different laboratories. Threedifferent methods were used to conduct routine ambient-porosity measurements onselected plugs. Porosities were obtained on the dry, extracted cores by use ofhelium and by saturating the core plugs with toluene and then brine. Thepurpose of these comparisons is to determine whether the porosity of the TravisPeak formation is influenced by the presence of brine and whether heliumporosities match those measured by saturation with a nonreactive porositiesmatch those measured by saturation with a nonreactive liquid. Next, ambienthelium porosity is compared with that measured at either 800 psi or netoverburden stress to determine the effect of stress on porosity. To address theeffect of stress on gas permeability, correlations are shown of Klinkenberg-corrected gas permeabilities, k, measured at near-ambientcondition and at net overburden stress. These correlations are greatly improvedafter cores with either induced or natural fractures are omitted. Correlationsare also presented for the Klinkenberg factor that are somewhat different fromthose previously presented in the literature and that are also improved afterpreviously presented in the literature and that are also improved after thefractured cores are omitted. Further, a correlation that relates brinepermeability to k is presented and compared with previously publishedliterature. previously published literature. For reservoir description andengineering applications, it is important to relate permeability to some rockproperty measurable from logs, such as porosity. Correlations are presented forthe Travis Peak formation relating k to porosity, both measured at net Peakformation relating k to porosity, both measured at net overburden stress. Thesecorrelations are greatly improved when fractured cores are omitted and furtherimproved when derived for specific environmental rock types. Ambient-Porosity Measurements A common method of measuring core porosity is to measure the grain volume byuse of helium and the bulk volume by immersion in mercury. This method isreliable when carefully performed and was used for most of the ambientporosities measured in this study. A second method is to determine the porosityby saturating the core with brine. This porosity could differ if the rock hasreactive clays. A third method is to determine the porosity by saturation withtoluene, which should not react with the rock. All the above methods were usedon a set of 41 core plugs from Well Howell No. 5. Fig. 2 compares porositiesmeasured by on with 15 % NaCl brine and with helium, with thebest-fit-reduced-major-axis (RMA) line as shown. Generally, the brine porosityis 0.25 porosity units less than the helium porosity. Fig. 3 comparesporosities measured with toluene and with helium again, with the best-fit RMAline as shown. The toluene porosity is generally about 0.6 porosity units lessthan the helium porosity, with the difference greater at low porosities. Incomplete liquid saturation of the low-porosity, porosities. Incomplete liquidsaturation of the low-porosity, low-permeability cores may be a factor, although extra care was used in this step. Our conclusion is that, because thehelium and brine porosities are essentially in agreement, porosities measuredwith porosities are essentially in agreement, porosities measured with heliumshould be reliable. Thus, for the overall data base, porosities were routinelymeasured with helium in dry, extracted cores. porosities were routinelymeasured with helium in dry, extracted cores. Porosity Measured at 800-psi Stress Porosity Measured at 800-psi Stress Among the four different servicelaboratories used for the routine core analyses, one had automated equipmentfor measuring porosity and permeability at net overburden stress on two studywells, Wells Howell No. 5 and SFE No. 2. In this system, the base routineporosity of each core was determined by measuring the PV with helium porosityof each core was determined by measuring the PV with helium while the core wasconfined in a core holder at 800-psi sleeve pressure. On the basis of sleevestiffness and conformance, the pressure. On the basis of sleeve stiffness andconformance, the porosities measured in this manner were reported to matchporosities porosities measured in this manner were reported to match porositiesmeasured at ambient (or unstressed) conditions. Fig. 4 compares porositiesmeasured with helium on 176 dry cores at ambient conditions with porositiesmeasured with helium at 800-psi sleeve pressure for Wells Howell No. 5 and SFENo. 2. Data are included only for those cores that were free of induced ornatural fractures. The best-fit RMA line shows a good match for low porosities(less than 5%). In the higher range, porosity is reduced about porosities (lessthan 5%). In the higher range, porosity is reduced about 1 porosity unit whengoing from ambient to 800-psi sleeve pressure. Evidence therefore suggests thata significant reduction in porosity my occur for these cores when confined by800-psi sleeve pressure in this particular system compared with ambientporosity. particular system compared with ambient porosity. SPEFE P. 310