| dc.description.abstract | Unconventional reservoirs, especially shales are drilled horizontally and hydraulically fractured for economical production. Proppant is carried and placed with the use of fracturing fluid to maintain fracture conductivity once the fracture is initiated. It has been observed that fracture conductivity degrades overtime due to several mechanisms such as proppant crushing, embedment, diagenesis and fines migration. To evaluate the performance of various proppants under reservoir stress conditions, American Petroleum Institute (API) proposed the proppant crush test, where load rate is maintained constant, to determine amount of fines generated in terms of “percent crush” by weight. However, the current standard procedure to measure crush resistance of proppants does not represent the realistic field conditions as it is based on measurement at high proppant concentration (4 lb/sq. ft), ambient temperature and dry condition.
To study the proppant behavior under stress, we conducted experiments at constant displacement rate to determine the critical pressure of a proppant pack with the aid of acoustic emission activity detection and particle size analysis. Critical pressure (σcrit) is defined as the pressure at which particles finer than the specified proppant size distribution start being generated as determined through particle size analysis. This is also the pressure for onset of grain crushing. In addition to acoustic emission activity and particle size analysis, images of proppant were captured as function of pressure. With the help of bulk density, grain density and images, porosity was calculated as function of pressure. Proppants used in this study are 20/40 mesh Ottawa sand and 20/40 mesh ceramic proppants: carboHSP, carboNRT and carboProp.
Our objective is to systematically study the effects of proppant concentration, fracture morphology, proppant type, cyclic loading, saturation and pre-test treatment on σcrit of the selected common proppants.
When proppant is tested dry and at ambient temperature, it is observed that loading rate, calculated from load data, and acoustic emission activity rate as function of load and time provide an indication of σcrit. σcrit depends on proppant concentration, proppant type and displacement rate. It is observed for dry 20/40 mesh Ottawa sand that higher proppant concentration is correlated with a higher σcrit. For 4 lb/sq. ft concentration, images of dry 20/40 mesh Ottawa sand captured as function of pressure show that crushing is non-uniform and primarily takes place at the steel-proppant interface. For 1 lb/sq. ft concentration, images show that crushing is uniform. Using images for the dry 4 lb/sq. ft concentration of 20/40 mesh Ottawa sand, we calculate a porosity reduction of 25% and proppant pack conductivity reduction of 14% due to compaction with pressure increase to 13000 psi. Similarly for the dry 1 lb/sq. ft concentration of 20/40 mesh Ottawa sand, we observe porosity reduction of 33% and conductivity reduction of 18% due to compaction to a pressure of 13000 psi. We conclude that higher proppant concentration corresponds to less porosity and conductivity reduction.
To understand the effect of fracture morphology on proppant crushing, the crush cell piston was modified so as to obtain differential proppant concentration as function of crush cell width. This study was motivated by observation of changes in fracture width on hydraulically fractured Tennessee sandstone under triaxial conditions in laboratory. Loading rate as function of load indicates that differential concentration of dry 20/40 mesh Ottawa sand shows relatively low crush resistance in comparison to uniform concentration of dry 20/40 mesh Ottawa sand. Particle size analysis performed at discrete pressures supports our inferences from loading rate as function of load. Images for dry 4 lb/sq. ft concentration of 20/40 mesh Ottawa sand indicate that crushing is non-uniform and primarily takes place at steel-proppant interface and the region where the proppant concentration is relatively low and stress is relatively high. For dry 1 lb/sq. ft 20/40 mesh Ottawa sand, crushing is non-uniform and primarily takes place in the region where the proppant concentration is relatively low and grain to grain stresses are relatively high. These observations indicate that fracture width modulation would play a role in differential crushing of proppant, thereby, affecting fracture conductivity.
Proppant crush tests were further extended to study the effect of cyclic loading on dry 20/40 mesh Ottawa sand. This study was conducted to investigate the effect of re-fracturing or well shut-in which would cause stress cycling on the already placed proppant pack in the reservoir. Cyclic loading experiment is conducted on dry 4 lb/sq. ft concentration of 20/40 mesh Ottawa sand after it is subjected to 15000 psi twice at constant displacement rate. Acoustic emission activity rate as function of time indicates that proppant pack does not undergo crushing in the 2nd cycle when the proppant pack has already been subjected to 15000 psi in the 1st cycle. Using particle size analysis conducted at the end of 1st and 2nd cycle, it is concluded that stress cycling does not significantly increase crushing.
The proppant crush cell was further modified to conduct the test on fluid saturated proppant packs. The apparatus was connected to metering pump to maintain a pore pressure of 50 psi on 4 lb/sq. ft concentration of 20/40 mesh Ottawa sand while stress on proppant pack was increased to 11000 psi at constant displacement rate. Imaging showed uniform fluid distribution in pores at 50 psi. Loading rate and acoustic activity rate as function of time and load were inconclusive. Particle size analysis indicated that 20/40 mesh Ottawa sand crushed, and, there was insignificant difference in particle size distribution at 11000 psi between dry 4 lb/sq. ft concentration of 20/40 mesh Ottawa sand and 20/40 mesh Ottawa sand exposed to fluid. However, there was significant effect of reduction in frictional resistance between grain-grain contact which led to a significant change in the compaction trend between wet and dry tests.
To investigate the effect of exposure to high temperature fluids on proppant crushing, 20/40 mesh Ottawa sand was exposed to distilled water for 3 days at 100 C. The 20/40 mesh Ottawa sand was then dried and crush test was performed on this heat and water treated proppant. Loading rate as function load indicates that treated 20/40 mesh Ottawa sand at 1 lb/sq. ft concentration shows significantly less crush resistance in comparison to test conducted on dry 20/40 mesh Ottawa sand. However, particle size distribution at 9000 psi of untreated and treated proppant at higher concentration of 4 lb/sq. ft showed relatively small differences. Loading rate data showed no significant difference between treated and untreated proppant.
In conclusion, we propose a new proppant crush test procedure which honors realistic field conditions i.e. low proppant concentration, uneven surface topography and fluids at elevated temperature. The standard API crush test overestimates fracture conductivity. Observing loading rate and acoustic emission activity rate as function of load while conducting test at constant displacement rate is recommended. | en_US |
