Carbonate Reservoir Fracture Treatment
Carbonate Reservoir Fracture Treatment: Acid or Propped?
Fractures are important conduits for oil and gas flow to producing wells. Fracture closure due to stress can catastrophically impair well productivity. Determination of fracture conductivity versus closure stress is critical for determining which treatment is needed in carbonate reservoirs. Core Lab performs laboratory tests that provide vital reservoir-specific information as to whether the reservoir should be acid or proppant fracture treated.Fracture Design Problems:
- Poor selection of fracture treatment for carbonate wells
- Excessive closure stress in acid fractured reservoirs can result in early production decline
- Proppant embedment reduces fracture width and lowers fracture conductivity
- Chalk influxes due to acid fracturing in carbonate rock may reduce fracture conductivity
Fracture Design Laboratory Solutions:
- Enhance production associated with improved fracture conductivity
- Relative comparison of acid and proppant conductivity on same sample leads to selection of best treatment
- Rock mechanics data is required in both acid and propped fracture treatment designs
Benefits of proppant treated fracture conductivity:
- Proper proppant selection can reduce embedment and enhance recovery
- High fracture conductivity serves as a ‘superhighway’ for conducting reservoir fluids to the wellbore
- Proppant holds the fracture open and provides a permeable path for the oil or gas to flow to the wellbore
Benefits of actid treated fracture conductivity:
- Stimulates low permeability carbonates by increasing conductivity
- Less expensive and less time involved than proppant fracturing
- Less complicated because no propping agent is used
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