The main objective of WP 2.05 was to enhance productivity of geothermal wells by applying stimulation techniques (hydrofracturing, thermal fracturing and acidizing).
Drilling cost is a major expense for upstream development costs. Reduction of drilling cost has a significant impact to the economy of the project. Well productivity is a key parameter for upstream development cost efficiency. Productivity of geothermal well is controlled by reservoir transmissivity and skin factor. Furthermore a successful stimulation job secures the overall project economy.
Prior to implementing the stimulation job, we had to consider candidate selection due to the nature of the reservoir, well geometry and expected improvement. Along with the earlier consideration, the cost benefit analysis should be conducted to get insights about the feasibility of the activity. There are different methods to stimulate wells. All methods enhance the permeability of the reservoir around the well. The largest pressure drop in the reservoir takes place just around the well, so increasing the permeability in this region significantly impacts the productivity (and/or injectivity) of a well.
The available methods are:
- Hydraulic stimulation. With this technology water is injected in the well with high pressure. This pressure causes the rock around the well to break (fracture). After pressure release the fracture closes again, and permeability can be retained either by putting a coarse material in the fracture before closing (propped fracture), or by movement of the rock along the fracture where the roughness of the fracture prevents the fracture to close completely after pressure release (shear fracture). Shear fractures cost less (no proppant, lower pressure required and only water injection without chemicals), but requires certain rock and stress characteristics. Propped fracs are very common in oil & gas industry.
- Chemical stimulation often uses acid to dissolve the rock or rock matrix around the well. the success of chemical treatment depends on the reactivity of the rock, the type of acid used and the amount of chemicals that can be injected into the well and make good contact with the rock.
- Thermal stimulation uses the fact that the rock shrinks when it is cooled. Rapid shrinking due to cold water injection leads to the formation of fractures.
Often combinations of the three methods are applied. The best type and method of stimulation depends on the local circumstances; when stimulating an existing well, also the status and design of the well will influence the choices that can be made.
Two master thesis implemented the scopes of this workpackage dealing with the thermal effect of fracturing on rock samples from typical Indonesian geothermal reservoir and with th acidizing.
Both thesis by Tulus Imaro (2017) and Prita Kandella Naymilton (2018) have been conducted in the geomechanical laboratories of TUD.
Additonal work has been dedicated to the collection of the rock samples used by for the experiments on regular field campaigns conducted during the lifetime of GEOCAP jointly with ITB and UGM. Localities such as Wayang Windu, Mount Salak and Flores have been visited in order to collect water and fluid samples. The most relevant results have been published in: