Controlled Fracturing for Increased Recovery

Fracturing provides a way to increase recovery of hydrocarbons from tight rocks. Marginal sandstone and chalk fields on the Norwegian continental shelf can potentially be stimulated and made economic through this technology. In North America, the recent gas and oil shale revolution is largely a result of combined horizontal drilling and multistage fracturing. This technology can however not be directly implemented in Norway: Environmental concerns call for better knowledge on where and how far hydraulic fractures would grow, and how to make the operations more efficient so that less water and fewer additives are required. Furthermore, water-injection for improved recovery frequently leads to fracturing; thus, the same knowledge is needed in order to properly design such operations. For future recovery of geothermal energy from large depth, controlled fracturing is a key to economic development. This project has given better knowledge of the mechanics of hydraulic fracture initiation and growth. This was done through development of a mathematical model calibrated on the basis of controlled laboratory experiments on relevant rock cores. The numerical model enables 3-dimensional simulation of fractures, accounting for temperature differences between the rock and the injected fracturing fluid and for fluid flow in the fracture and the surrounding rock formation. Complex fracture paths and multiple fractures can be studied. In the first two years of the project, the emphasis of the project was on the development of the 3D numerical hydraulic fracturing model, MDEM (modified discrete element model). By parallelization of the code and optimization of algorithms, the computing time could be strongly reduced. One advantage of a numerical fracturing model is the ability to describe non-planar fracture growth in heterogeneous rock formations, or spatially varying stress field (in the vicinity of a producer or in the presence of other fractures), which has been demonstrated by 2D MDEM. Two large-scale hydraulic fracturing laboratory test have been carried out at TU Delft (partner in this project) by pressurizing a deviated hole in a cement sample. As confirmed by X-ray computed tomography (CT) images, due to stress redistribution around the hole, a fracture is initiated parallel to the hole and bends into the horizontal direction, perpendicular to the minimum far-field principle stress, as it propagates further away from the hole. By applying 3D MDEM, it was possible to qualitatively reproduce the experimentally observed fracture bending around a deviated hole. A new true triaxial cell for large-scale rock testing was installed at SINTEF in 2017. This cell has a unique design that was developed by SINTEF in collaboration with MTS in the US. Different from other true triaxial cells, sealed, cylindrical samples can be subjected to triaxial stresses, which makes it possible to carry out hydraulic fracturing tests and measure the fracture permeability. A first hydraulic fracturing test was carried out in Q1 2018 with a cement sample and this has been followed by four fracturing tests with tight sandstones. The target with these experiments are to investigate the effect temperature reduction around the hole has on the fracturing pressure. With the help of the PhD student in the project, large progress was made to describe thermal effects on hydraulic fracturing and fracture flow. Amongst others, it was investigated how thermal stresses affect the flow through a network of fractures for geothermal applications. In another ongoing study, it is investigated if thermal effects can efficiently be employed to avoid high injection pressures at water-injection wells by reducing both fracture initiation and fracture propagation pressures. The work is done by SINTEF and NTNU in cooperation with industry partners and international researchers among others in Canada, Netherlands, Wales and Cyprus.

Prosjektet har bidratt til økt forståelse av hydraulisk oppsprekking i lavpermeable formasjoner. Denne forståelsen er viktig for å designe trygge, effektive og kostnadsbesparende sprekker innen reservoar stimulering og økt oljeutvinning. Programvaren utviklet i dette prosjektet kan også bli brukt innen geotermisk brønn design. Hovedleveransen fra dette prosjektet er programvaren MDEM3D, som gjør det mulig å studere hydraulisk oppsprekking i 2D/3D uten å forhåndsdefinere sprekken. MDEM3D blir distribuert som åpen kilde programvare. Dette gjør det mulig for universiteter og andre forskningsmiljøer å bruke programvaren til utdanning og økt forståelse. Bruken av en triaksial ramme med tre spenningsretninger og utviklingen av prosedyre og utstyr for hydraulisk oppsprekkingseksperimenter med væsketemperatur kontroll og ultrasonisk avbildning av sprekkprosessen er unik. Resultatene og videre bruk av eksperimentelt utstyr kan gi betydelige bidrag til forståelsen av hydrauliske oppsprekking.

Reliable 3D simulation of hydraulic fracturing is a key enabler for economic development of tight reservoirs on the NCS and increased recovery from mature fields during secondary and tertiary recovery. A particular challenge is to take into account the full complexity of the rock formations, well geometry and stress orientation. This project consists of two interconnected work packages (WP). WP1 addresses development, calibration, and validation of a fully hydro-thermo-mechanically coupled 3D hydraulic fracture model. The model builds on an existing fluid-coupled 2D model based on a hybrid finite element/discrete element method. It will permit simulation of complex fracture growth from horizontal or deviated wells into naturally fractured formations exhibiting non-elastic stress-strain response. In this sense, the new model will surpass the capabilities of existing hydraulic-fracture simulation tools. WP2 addresses multi-scale fracturing experiments from cm to field scale. Laboratory tests with small sandstone and chalk samples will be used for studying fracture-initiation processes. Hydro-mechanical coupling during fracture growth under in-situ stress conditions, as well as fracture twisting around deviated holes will be studied with larger rock samples (up to 40cm diam.). Finally, the laboratory test data is complemented by field-test data from a scientific hydraulic-fracture and mine-back project in a deep mine. The experimental results will provide an ultimate benchmark for the new hydraulic fracture model. Application areas include fracturing for increased recovery in tight sandstone and chalk reservoirs on the NCS and controlled thermal fracturing to optimize the efficiency of water injection. The impact of the project will be an improved understanding of thermo-hydro-mechanical mechanisms and primary control parameters of hydraulic-fracture initiation and propagation in rocks representative of target reservoirs with the help of dedicated experiments.