Abstract
In Ecuador there are mature subhydrostatic sandstone reservoirs that have lateral water drive or that are located close to the oil/water contact. With time the water saturation has increased and water breakthrough is common in high-permeability streaks, causing the water cut to increase substantially. In these fields the challenge is to increase oil production and recoverable reserves through stimulation without increasing the water cut. For this reason, until recently, hydraulic fracturing was not considered a viable option. Although conventional acid treatments increased oil production, they also increased the water cut, sometimes making the production uneconomical. To overcome this, either acid treatments with diverter stages are used to selectively stimulate the oil-saturated intervals or relative permeability modifiers are used to limit water production. However, these have not proved to be reliable solutions.
To hydraulically fracture wet producers, a linear fracturing fluid was introduced. The fluid is formulated with low concentrations of a disproportionate permeability modifier (DPM) or a relative permeability modifier (RPM), and guar polymer creates a pseudoviscoelastic disproportionate permeability modifier (VDPM). The VDPM reduces the effective permeability to water in the fracture faces, and the relatively low viscosity of the fluid limits the net pressure and the risk of fracture height growth, without compromising proppant transport. The viscosity and leakoff control of the fluid can be adjusted as a function of the formation properties, leak-off and stress profile.
The first hydraulic fracturing campaign of eight wells using the VDPM fluid was in the Hollin formation, which has a strong aquifer below it and in some cases a lateral aquifer. Typically, the upper section produces oil and the lower section is water saturated; the distance between the two sections is 5 to 25 ft. On average the oil production increased threefold and the water cut was not increased after the fracturing job, in spite of the proximity to the wet zone. While the results of matrix treatments with RPMs in wet producers are notoriously mixed, hydraulic fracturing with a VDPM fluid has proved a reliable means to selectively increase oil production in the presented case. This has had a very positive impact on the economics and reserves recovery in these marginal mature fields.
