Mahmoud Asadi

Static/Dynamic Settling of Proppant in Non-Newtonian Hydraulic Fracturing Fluids

Experimental data and analysis are presented on the static and dynamic settling of proppant in non-Newtonian hydraulic fracturing fluids. A unique High Pressure Simulator (HPS) that closely models reservoir hydraulic fractures is used to study proppant-settling behavior under both static and dynamic conditions. A system of optical fibers and Light Emitting Diodes (LED) acting as a vision system were imbedded in the platens representing fracture walls for quantification of proppant settling under various fluid injection conditions. Hydraulic fracturing fluids tested include slurries of 35 lb/Mgal guar linear gel with 6 ppg 20/40 mesh sand, 60 and 40 lb/ Mgal hydroxypropyl guar (HPG) linear gels both with 4 ppg 20/40 mesh sand, and borate-crosslinked 35 lb/Mgal guar gel with 7.3 ppg 20/40 mesh sand. Test conditions cover a shear rate range of 60 to 120 sec -1 . In addition, the effect of an enzyme breaker on proppant settling rate under static conditions was studied. Tests were conducted with a fracture gap width of 0.375 in, under ambient conditions. Analysis of the dynamic condition indicates that proppant settling within the linear and crosslinked gels investigated is non-linearly dependent on shear rate. Under static conditions, linear gel slurries settled 40 to 50 times faster than the crosslinked gel containing breaker. It is also shown that an increase in shear rate induces proppant settling in borate-crosslinked 35 lb/Mgal guar gel while it reduces the settling rate in 35 lb/Mgal guar linear gel.

Proppant Transport Characterization of Hydraulic Fracturing Fluids using a High Pressure Simulator Integrated with a Fiber Optic/LED Vision System

Slurries of selected hydraulic fracturing fluids such as 40 lb/Mgal HPG linear gel and borate-crosslinked 35 lb/Mgal guar gel were evaluated to characterize their proppant transport properties using 6 ppg 20/40 mesh sand. In addition, fluids such as water, 40 lb/Mgal HPG linear gel, and borate-crosslinked 35 lb/Mgal gel were evaluated to determine their relative ability to remove the fluidized layer lying along the top of settling proppant bed and to subsequently erode the more compacted layers below. The experimental study utilized a unique high pressure parallel plate flow cell, simulating a downhole fracture, integrated with a vision system of fiber optic and Light Emitting Diodes (LED). The vision system was used to quantify proppant transport characteristics and to study various aspects of proppant transport including bank buildup. Selected hydraulic fracturing slurries were pumped into a high-pressure flow cell having a 0.375 fracture gap width. The slurries were pumped through 3000 ft of 1.188 in. ID coiled tubing, 500 ft of 2 in. ID heat exchanger, and a 2.75 in, inlet manifold that could be configured to represent a wellbore having various perforation arrangements. Slurries were pumped for a selected time at a fixed shear rate in the range of 60 to 120 sec -1 to observe the rate of bed sedimentation. These initial slurry injections were followed by various fluids and slurries to assess the degree of compaction within the previously deposited bed. Data analysis indicates that slurry sedimentation in polymer solutions follows a non-linear relationship with time at a fixed shear rate and that perforation configuration affects proppant transport, bed height growth, and proppant bed wash off near the wellbore. It is also shown that bed height growth is not directly proportional to shear rate. Results from the current study also show that the fluidized layer along the top of the proppant bed is easily removed by a clean crosslinked gel. However, erosion of the more compact underlying layers was found to depend on a number of factors. Among these factors are the type of slurry which formed the deposit, the type of fluid or slurry being used in an attempt to erode the bed, and the total pumping time devoted to eroding the bed.

Convection/Encapsulation in Hydraulic Fracturing

New information is presented on the phenomena of proppant convection and encapsulation in the hydraulic fracturing treatments. To fully comprehend this phenomenon, a unique parallel plate flow cell is used which mimics field conditions and is integrated with a fiber optic vision system. The vision system is used to enable the visualization of proppant convection/encapsulation within the fracture. The experimental study involves pulse injection of both 6 and 10 ppg 20/40 mesh sand slurry of 35 lb/Mgal borate-crosslinked guar into various fluid media under static condition. Data analysis shows that proppant convection/encapsulation is a density/viscosity related mechanism. It is indicated that encapsulation (downward motion of proppant) dominates particle settling in a low viscous medium such as water or linear gel. This phenomena, however, diminishes as viscosity of the lower medium increases. In other words, particle settling within the slurry cluster dominates proppant encapsulation and hence convection. Therefore, it is ascertained that there exists an optimum viscosity value for a given range of density where the influence of density to form encapsulation and convection is inhibited. According to this study, a medium viscosity of 40,000 cp is suffice to sustain a 10 ppg 20/40 mesh sand 35 lb/Mgal borate-crosslinked slurry and to prevent development of proppant encapsulation/convection. In addition, to better understand the behavior of convection/encapsulation in hydraulic fracturing, the effect of fracture gap width and polymer concentration on the encapsulation/convection development investigated and presented.

A New Empirical Correlation to Predict Apparent Viscosity of Borate-Crosslinked Guar Gel in Hydraulic Fractures

Successful design and completion of the hydraulic fracturing treatments depend on accurate estimation of fracturing fluid viscosity under in-situ conditions. However, the viscosity of the widely used borate-crosslinked guar gel is extremely difficult to measure due to its dependence on temperature, wellbore shear pre-conditioning, fracture shear rate, and pH. Furthermore, these properties require that the fluid mixing, pumping, and characterization to be performed under representative field conditions. Because of these complexities, there is no reliable correlation available in the literature to relate borate-crosslinked guar gel viscosity with its dependent properties. Therefore, in the present study, borate-crosslinked guar gel rheology measurements, obtained with a field scale High Pressure Simulator (HPS), are used to develop a correlation. The empirical correlation is developed on the basis of method of reduced variables, whereby shear stress-shear rate data at different temperatures are combined into one curve at a reference temperature T o . Present study extends this method to include the effect of shear history on the rheology of guar polymer crosslinked with borate ions. In the paper, this technique is used to generate master curves of reduced apparent viscosity-shear rate for borate-crosslinked guar at each pH. Furthermore, the developed empirical equations relate apparent viscosity of crosslinked guar to fluid temperature, shear history and shear rate. The developed correlation is presented in a simple form. The correlation will be helpful for comparison of model calculated values with the laboratory measured viscosities, as the laboratory values are not supported by a field scale fluid characterization equipment.