Injection fracturing in a densely spaced line drive waterflood - The Halfdan example

Induced fractures are actively steered along horizontal water injection wells in a densely spaced line drive water flood, thus ensuring efficient sweep and minimising random connections between parallel producer and injector wells. Due consideration of flow-induced changes to the stress field reduces the risk of premature water breakthrough during injection at fracturing conditions. The paper differs from other works concerning flow-induced stresses by focussing on the application of this phenomenon and the practical experiences from a field-wide implementation. A technique, named "Fracture Aligned Sweep Technology" (FAST), has been implemented on the Halfdan chalk field in the Danish North Sea with horizontal wells drilled at 600 ft lateral spacing in a parallel pattern of alternating producers and water injectors with 10,000 to 15,000 ft long reservoir sections. Fracturing of the injector wells is the key to the process of voidage replacement, due to the low mobility of water compared to oil and gas. FAST uses the fact that fluid flow in rock of low permeability affects reservoir stresses. Before propagating a fracture, the prevailing pressure field is manipulated through a period of injection below fracture propagation pressure and simultaneous production from the neighbouring wells. Numerical simulation of fracture propagation shows that seepage forces strongly influence the propagation direction when an injection fracture propagates in a laterally varying pressure field. At slow propagation rates the pressure diffusion from the fracture itself causes alignment of the fracture with the injection well, i.e. the technique works because the injection rates are actively controlled. Confinement of injection fractures along horizontal injector wells is verified by production data from areas where FAST has been implemented. After 2-3 years of injection, water breakthrough to the neighbouring producers has not occurred. A positive production response to the high rate injection is demonstrated by increasing oil rate and decreasing gas-oil ratio.