Increasing Production and Reserves Through Fracture Stimulation in Challenging, Multi-Zone, Vertical Well Completions

Abstract

Abstract The oilfield is scattered with wells that did not initially require fracture stimulation for economic production but could now benefit from hydraulic fracturing to improve well performance, increase reserves and accelerate production. Many of these wells are producing from multiple perforated intervals through completions with production tubing that were not designed for fracturing and cannot be worked over. This paper is a case study of a North African oil field producing from two Ordovician sands, one with a higher permeability (5 mD) and one with a much lower permeability (0.5 mD), where significant benefit was achieved by fracture stimulating one or both intervals in two wells, despite completion and operational limitations. The design of the fracture treatments and the methods used to try to achieve stimulation of the selected intervals are discussed in detail. Pre-fracture well test, PLT and production history data were all used to history match the pre-frac production using reservoir simulation modeling, to determine which intervals were contributing and to estimate the kh of each interval. Fracture design models were made using these results and different scenarios were evaluated, including the possibilityto cover both intervals with a single fracture and the effect on fracture placement of different fracture initiation locations. Contingencies were developed based on these fracture design simulations to use degradable diverting materials and proppant plugs if needed. Post-fracture production, well test and PLT results were analyzed to determine the fracture properties, the connected kh and if all stimulation objectives were met. In the first well, by-passing a high skin in the lower interval was the primary goal, but both intervals would benefit from fracture stimulation. The treatment had to be pumped down tubing and performed in a single operation. Temperature logging after the minifrac showed that both intervals were broken down and could be treated with a single large, propped fracture treatment, avoiding the use of mechanical diversion. In the second well, only the tighter upper interval was targeted, but a tubing completion with a slotted liner across the target interval made zonal isolation of the lower interval challenging. In both cases, results were above expectations and showed a stimulation of both the targeted intervals and an increase in the connected kh compared to log and well test data. This is common in fracturing of layered heterogeneous reservoirs, where a perforated completion does not contact all the potential thin pay layers. This paper describes real-world success stories of fracturing with multiple intervals open through an existing completion that is not designed for fracturing. It highlights the potential for fracturing in many existing reservoirs to improve productivity, increase reserves and improve acceleration, without having to recomplete the well.