Hydraulic Fracture Design
Optimization of Hydraulic Fracture Design Using Geomechanics Tests
Hydraulic Fracturing Plays an Important Role in Productivity Improvement
Calculation of fracture height, width, and length are required for the optimization of the fracture design. Among data requirements to optimize hydraulic fracture design, mechanical properties (Young’s modulus, Poisson’s ratio, fracture toughness and poroelastic constant), fracture gradient, and distribution of minimum horizontal stress are important factors that control or influence fracture geometry.
- Unconfined fracture height growth causing screen-outs and inefficient proppant placement
- Proppant embedment reducing fracture width and lowering fracture conductivity
- Excessive closure stress resulting in early production decline
- Unknown fracture azimuth leading to poor well placement
Data for Optimized Fracture Design
The data required for optimized fracture designs are the usual reservoir parameters of porosity and permeability, along with a variety of rock mechanics parameters at multiple vertical locations in the pay zone and in the subjacent and superjacent barriers (both) which must arrest the vertical fracture growth. The rock mechanics data include:
- Static and dynamic values of Young’s modulus and Poisson’s ratio
- Static values of fracture toughness and Biot’s poroelastic constant
- Calculated values for minimum horizontal stress versus depth
Triaxial Compressive Test
To characterize mechanical properties of the reservoir rocks, triaxial compressive tests are performed at a range of confining pressures. The triaxial compressive tests are commonly used to simulate in-situ stress conditions of the reservoirs and provide compressive strength and static values of elastic constants (e.g., Young’s modulus and Poisson’s ratio). Since there is a significant difference between static and dynamic values, it is important to calibrate dynamically derived mechanical properties to the statically measured values that better represent the in-situ reservoir rocks.
Strength of brittle materials is governed by the presence of small cracks present within grains and at grain boundaries. A fracture will propagate when the stress intensity factor (e.g., KI for opening mode crack) reaches the critical stress intensity factor, KIC, also known as fracture toughness. Therefore, the fracture toughness is a measure of the resistance of the rock to crack propagation. Some fracture design programs require fracture toughness to predict fracture height.
Proppant Embedment Test
Proppant embedment is an important problem today because of fracturing stimulation treatments performed in softer formations. Unlike well consolidated rocks, embedment can be as high as several proppant-grain diameters in softer formations. Proppant embedment can reduce fracture width from 10% to 60% with subsequent reduction of productivity from oil and gas wells. Proppant selection can help reduce embedment and enhance recovery.
Brinell Hardness Test
The fracture hardness value is an important factor due to industry trends to fracture softer, weakly consolidated and higher porosity formations. The Brinell Hardness is a measure of the resistance of the rock to indentation and has a direct implication for proppant embedment problems.
Knowledge of fracture azimuth is important in placement of horizontal wells and determining the well locations in tight formations for drainage optimization and drive/sweep efficiency of water flooding or EOR. The primary purpose of the measurement of sonic velocity anisotropy is to determine the direction of maximum horizontal stress and hence the optimum fracture azimuth. Fracture azimuth can be determined with a field proven method (over 50 wells) that is both cost effective and reliable