Phillip M. Halleck

Permeability reduction of a natural fracture under net dissolution by hydrothermal fluids

[1] Flow-through tests are completed on a natural fracture in novaculite at temperatures of 20� C, 80� C, 120� C, and 150� C. Measurements of fluid and dissolved mass fluxes, and concurrent X-ray CT imaging, are used to constrain the progress of mineral dissolution and its effect on transport properties. Under constant effective stress, fracture permeability decreases monotonically with an increase in temperature. Increases in temperature cause closure of the fracture, although each increment in temperature causes a successively smaller effect. The initial differential fluid pressure-drop across the fracture increases by two orders of magnitude through the 900 h duration of the test, consistent with a reduction of an equivalent hydraulic aperture by a factor of five. Both the magnitude and rate of aperture reduction is consistent with the dissolution of stressed asperities in contact, as confirmed by the hydraulic and mass efflux data. These observations are confirmed by CT imaging, resolved to 35 microns, and define the potentially substantial influence that benign changes in environmental conditions of stress, temperature, and chemistry may exert on transport properties. INDEX TERMS: 5104 Physical Properties of Rocks: Fracture and flow; 5114 Physical Properties of Rocks: Permeability and porosity; 5194 Physical Properties of Rocks: Instruments and techniques; 5134 Physical Properties of Rocks: Thermal properties; 8135 Tectonophysics: Hydrothermal systems (8424). Citation: Polak, A., D. Elsworth, H. Yasuhara, A. S. Grader, and P. M. Halleck, Permeability reduction of a natural fracture under net dissolution by hydrothermal fluids, Geophys. Res. Lett., 30(20), 2020, doi:10.1029/2003GL017575, 2003.

A fractal model for predicting permeability around perforation tunnels using size distribution of fragmented grains

Abstract A methodology to predict the permeability distribution around the perforation tunnels is presented. The resulting equation describes the permeability by modeling the distribution of grain fragments and by fractal concepts. The equation allows the calculation of permeability from the particle size data obtained from the rock material surrounding the tunnel. The fractal model constructs a flow equation for an incompletely fragmented porous media by describing the geometry and size of the pores using the grain size distribution and setting up a hierarchical scaling concept for fragmentation to eliminate the extra porosity of a completely fragmented fractal porous medium. Predicted permeabilities (as compared to measured ones for the 2.4- and 5.1-MPa underbalance shots) using the developed equation showed that the predictions are successful at almost all radial positions around the tunnel. Relative errors between predictions and measurements increase towards the tunnel wall. This may be attributed to the unfavorable particle shapes, which are assumed to be spherical in the model, and aspect ratios of fragmented grains. It also was observed that predictions for higher underbalance test are better due to better surge flow cleaning, which eliminates most of the small particles affecting the calculations due to their unfavorable shapes. The developed equation provides an easier and cheaper way of mapping the permeability distribution around the perforation tunnels compared to the viscous fluid injection and pressure transient measurement techniques. The proposed technique can be valuable to obtain permeability distribution data for near-wellbore modeling of specific formations perforated at laboratory conditions and also for the assessment of the permeability damage created by different charge and gun designs.

Experimental investigation of surge flow velocity and volume needed to obtain perforation cleanup

Abstract Underbalanced perforating has been practiced for a number of years to minimize formation damage in and around the perforation tunnel. High transient flow rates are thought to sweep crushed rock and charge debris into the wellbore, leaving a clean tunnel. With sufficient underbalance, formation damage surrounding the tunnel is also reduced. In practice, the minimum underbalance and the volume of surge flow required per perforation become part of the completion design. Recent theoretical studies predict that most of this cleanup process occurs early in the transient flow process, suggesting that large transient flow volumes may not be needed. Using a modified standard test procedure from the American Petroleum Institute, transient flow rates into perforations in Berea Sandstone have been measured, varying underbalances and surge flow volume. Flow rate, instantaneous core flow efficiency (CFE) and local flow velocity are reported as a function of time, starting ∼ 0.5 s after firing. The results indicate that (1) where sufficiently high underbalance is used, CFE reaches its maximum value within the first 1–3 s. Subsequent transient flow and quasi-static flow do not further increase flow performance. (2) Tests with large and small surge flow volumes behave similarly. This observation is consistent with early cleanup. (3) At lower underbalance, CFE was reduced as expected. However, the ultimate value was still obtained within the first second of transient flow. The remainder of the surge flow did little to improve performance.

Mapping of Permeability Damage Around Perforation Tunnels

We have investigated porosity arid permeability damage around perforations using a combination of transient analysis and X-ray CT. The method applied allowed us to perform the entire experiments on samples under simulated in-situ stress conditions and to map variations in permeability along the length of the core as well as with radial distance from the perforation. Berea (10.2-cm (4-in.) dia) cores saturated with low-viscosity silicone oil were perforated using conventional-shaped charges (6-g HMX) and API RP43 procedures by using 6.88-MPa (1000-psi) effective stress and 5.16-MPa (750-psi) and 2.61-MPa (350-psi) underbalance. Low-permeability Torrey Buff Sandstone was also perforated using 5.16-MPa (750-psi) underbalance. After sufficiently flowing the perforations, higher-viscosity silicone oil was injected. The movement of fluids was tracked using X-ray CT to measure the local velocity of the viscous fluid front at different locations along the perforation. Results of these tests were compared in terms of permeability and porosity damage. Quantitative analysis on Berea cores show, for the specific charge and test conditions used, that damage extends approximately 2 cm (0. 78 in.) from the center of the perforation. Comparison of tests performed with 2.41-MPa (350-psi) and 5.16-MPa (750-psi) underbalance show a clear increase in permeability near the tunnel wall at the higher underbalance. A zone of somewhat-reduced permeability exists at approximately 1.7 cm from the perforation center in the latter case. Porosity profiles calculated show that porosity is almost uniform out from the tunnel and there is no compacted zone near the tunnel wall in liquid-saturated cores. However there is a high-porosity zone from the tunnel wall out about 2 mm. This may be due to a region of circumferential partings and small cracks that lead to high porosity or due to the possible artifacts discussed in the paper. Qualitative results have also been obtained for a tight sandstone for which underbalance was insufficient to remove debris from the perforation tunnel. CT images reveal that the plugged tunnel acts as a conduit for fluid flow, showing that the plugging material has significantly higher permeability than the surrounding rock.

The effects of annealing and aluminum substitution on the elastic behavior of alkali silicate glasses

Abstract In an effort to better understand the structural dynamics of glasses, we have measured the second-order adiabatic elastic moduli, and their pressure and temperature derivatives, of a sodium silicate and a sodium alumino-silicate glass. In both cases, we find increasing compressional and decreasing shear wave velocities with pressure. Substantial hysteresis is present in both glasses, especially in the shear-wave velocities. We also have studied the effects of annealing by measuring the elastic properties before and after annealing. We find that, in addition to previously observed increase in density and moduli, the pressure and temperature derivatives also increase, or become less negative. Comparison of the two glasses, which have nearly the same oxygen packing densities and unoccupied volumes, shows that substitution of aluminum for silicon increases the elastic moduli substantially. Shear modulus increases are accounted for by elimination of non-bonding oxygens, but bulk and Youngs modulus changes are associated either with bond strengths in the network-forming tetrahedra, or with steric hindrance effects. In contrast, aluminum substitution does not affect the pressure derivatives of the moduli. The pressure derivatives are proportional to the unoccupied volume fraction in the glass, which is a measure of ion packing efficiency. This suggests that packing efficiency alone controls moduli pressure derivatives in alumino-silicate glasses.

Comparison of shaped-charge perforating induced formation damage to gas- and liquid-saturated sandstone samples

Abstract This paper presents the results of two perforation tests performed to investigate porosity and permeability damage in the crushed zone due to shaped-charge perforating in gas-saturated and liquid-saturated Berea sandstones. Perforation experiments were conducted using conventional 6-g HMX explosive charges on 10-cm (4-in.)-diameter Berea sandstone cores saturated with either low viscosity silicone oil or nitrogen gas. During perforating, 6.9 MPa (1000-psi) effective stress and 5.2 MPa (750 psi) underbalance conditions were applied. Perforation flow tests before and after perforating were performed according to API-RP43 procedures. Permeability around the perforation tunnel was mapped using a viscous fluid injection method. Porosity was mapped by employing X-ray CT (computerized tomography) scanning and image quantification. Computation of permeability and porosity, as well as analysis of the images, show that, for the specific charge and test conditions used, the type of fluid saturating the rock results in significantly different perforation geometries and damage under the same operating conditions. In the gas-saturated core, a zone of compacted rock about 6 mm thick and with 6–8% less porosity than the average rock surrounded the perforation tunnel. Near the entrance hole, the compaction was more severe when compared to other locations along the tunnel. The liquid-saturated core had a clean tunnel with a larger diameter and deeper penetration than the gas-saturated core. The liquid-saturated rock had no compacted zone. Permeability damage extended approximately 1.5 cm from the center of the perforation in the liquid-saturated core. The particle size distribution data in the crushed zone, for both cases, show that there was grain fragmentation in the rock. However, it was more severe in the gas-saturated core.

The Evolution of Permeability in Natural Fractures - The Competing Roles of Pressure Solution and Free-Face Dissolution

Abstract Fracture permeabilities are shown surprisingly sensitive to mineral dissolution at modest temperatures (c. 20°–80°C) and flow rates. Net dissolution may either increase or decrease permeability, depending on the prevailing ambient THMC conditions. These behaviours have important ramifications for constitutive laws for flow and transport. Flow-through tests are completed on a natural fracture in novaculite at temperatures of 20°C, 80°C, 120°C, and 150°C, and on an artificial fracture in limestone at 20°C. Measurements of fluid and dissolved mass fluxes, concurrent X-ray CT and imaging, and post-test sectioning and SEM are used to constrain the progress of mineral dissolution and its effect on transport properties. For the novaculite, under constant effective stress, fracture permeability decreased monotonically with an increase in temperature, with fracture permeability reducing by two-orders-of-magnitude over the 900 h test. For the limestone, an initial decrease in permeability over the first 935h of the test, switched to a net increase in permeability as distilled water was subsequently circulated for the final 500h of the test.

Perforating Unconsolidated Sands: An Experimental and Theoretical Investigation

This paper describes the experiments and modeling undertaken to develop a perforating strategy for unconsolidated sand formations. The experimental program consists of single-shot perforations in cylindrical samples of three sizes (4-, 7-, and 10.5-in. diameters) of unconsolidated sand saturated with kerosene at residual brine saturation and placed under a high effective stress in a pressure vessel. The amount of sand produced during perforating, as well as that produced during post-shot flow, is measured, and a video probe is used to observe post-shot sand production. Computerized tomography (CT) scan equipment is used to map the geometry of the cavity. The sand features behind the casing are examined and interpreted, providing essential insights into the interaction of the charge with the sand sample. A theoretical model of the failure mode of unconsolidated sands has been developed that supports the observations. The main results so far from this study are as follows. Perforating does not generate a tunnel in unconsolidated sands. The intense pressures set up by shaped-charge penetration in the weak sand sample fluidize it and cause the initial tunnel to collapse. Production flow creates a dilated zone around the tunnel entrance. At a critical flow rate, a large volume of sand is produced and the sample collapses. This occurs when flowing out of a hole in an unperforated sample as well. The implications of these results for future work and for field applications are also addressed here.

Evaluation of the Relative Importance of Parameters Influencing Perforation Cleanup

Completion of cased and cemented wells by shaped-charge perforation results in damage to the formation, which can significantly reduce well productivity. Typically, underbalanced conditions are imposed during perforation in an effort to remove damaged rock and shaped-charge debris from the perforation tunnel. Immediately after the shaped-charge jet penetrates the formation, there is a transient surge of fluid from the formation through the perforation and into the well bore. Experimental evidence suggests that it is this transient pressure surge that leads to the removal of damaged rock and charge debris leaving an open perforation tunnel. We have developed a two-stage computational model to simulate the perforation process and subsequent pressure surge and debris removal. The first stage of the model couples a hydrocode with a model of stress-induced permeability evolution to calculate damage to the formation and the resulting permeability field. The second stage simulates the non-Darcy, transient fluid flow from the formation and removes damaged rock and charge debris from the perforation tunnel. We compare the model to a series of API RP43 section 4 flow tests and explore the influence of fluid viscosity and rock strength on the final perforation geometry and permeability.

Effect of Pore Fluid Type on Perforation Damage and Flow Characteristics

Perforation tests were performed to investigate porosity and permeability damage caused by perforating in gas-saturated vs. liquid-saturated conditions. The liquid-saturated core had a cleaner tunnel with a larger diameter and deeper penetration than the gas-saturated core.