One of the objectives of hydraulic fracturing in shale reservoirs is to maximize the Stimulated Reservoir Volume (SRV) or fracture network. Below we examine the use of CO2 pre-pads to increase productivity by enhancing dendritic fracture networks and maximizing the effectiveness of these un-propped fractures.
There are four major benefits to this technique:
‘Thermal shock’ from rapid cooling of the rock reduces stress, and enhances shattering of the rock and SRV’;
Low fluid viscosity adds to complexity of the fracture system;
Capillary trapping of the water is reduced, enhancing fracture effectiveness;
CO2is sequestered by kerogens, and does not alter wettability.
Thermal Shock in Shales (URTeC 1620617)
Ferus Inc. has performed thermal modeling that suggests the liquid CO2 enters the formation at the end of a 5,000 m wellbore at approximately 0 °C;
The cold fluid entering the hot formation causes stress reduction through shrinkage as well as thermal fracturing;
Reservoir modeling work has shown a 150% increase in gas production in early time, and a 20% increase over the longer term.
The picture shows thermal fracturing perpendicular to the main created fracture surface (URTeC 1620617):
Low Viscosity of the Injected Fracture Fluid (IPTC 17594)
Low viscosity fluids reduce the breakdown pressure as well as enhance the complexity of the SRV;
Liquid CO2 has a viscosity of 0.1 cp, about 10% that of water at surface and approx. 2-3% of polymer slickwater;
This low viscosity fluid will readily enter any natural fractures or fissures, kerogens and tight porosity – all common in shales;
When the CO2 warms to a gas, expansive forces will enhance the SRV and fracture growth.
Low Water Saturation and Capillary Trapping of Water
Shales are extremely low porosity and under-saturated in water (typically <15% Sw);
Kerogen rich shales also tend to be oil-wet;
Injecting high volumes of water will be naturally imbibed, altering the saturation of the rock as well as relative permeability to hydrocarbon flow;
This is the main reason why load fluid recoveries are very low in shales, typically <20%;
If CO2is injected into the tight fissures and pore space first, it will reside in the formation behind the water from the main frac as a ‘super-critical fluid’. When flowback starts, the phase change to a gas will help push the water back to the wellbore, aiding recoveries. Most importantly – near the frac tips the Sw has not been altered.
CO2 is Sequestered and Does Not Alter Wettability (SPE 134583)
CO2has an natural affinity (adsorption force) with kerogen and oil-wet surfaces;
When CO2enters the kerogen it releases the hydrocarbon and permanently occupies that space, becoming sequestered – 60 to 97% of the CO2 injected will be stored in an adsorbed state;
Perhaps more importantly, the oil-wet nature of the organic surfaces is not changed, saturations are not changed, thus enhancing hydrocarbon flow thru the tight pore space and fissures.
Secondary Benefits – Mineralogical Reactions
Shales tend to be clay rich, often softer formations than pure sandstone;
Water reactions with the clays may cause damage thru:
Softening along the fracture face, embedding proppant;
Fines migration into the proppant pack, reducing conductivity;
CO2/ water mixtures naturally have a lower pH, reducing these reactions by buffering the clays.
Where have Pre-Pads Been Used and What Were the Results?
Pre-pads have been shown to enhance productivity in numerous reservoirs; contact us for specific case studies.