Abstract

Most fracture stimulations are carried out using either water or oil-based fluids. Water-based systems have proven to be non damaging except in undersaturated formations. Project economics show that water-based fluids are less expensive and safer. Water-based fracture treatments at Crestar are normally conducted as the first phase of well stimulation. In undersaturated reservoirs, though, post fracture load fluids can be lost due to reservoir plugging, making the well uneconomic. Fortunately, the well is not permanently damaged and can be re-fractured with an oilbased fluid.

Two wells, one producing oil from the Glauconite formation (14–20-39-4W5) and the other a gas producer completed in the Viking formation (4-8-39-8W5) were fractured with a cross-linked water-based fluid. Both wells failed to respond to the stimulation, had very low load fluid returns, and were considered for abandonment. An extensive review of the reservoir and offset wells, plus a rigorous reinterpretation of the 14–20 & 8–4 well logs, suggested the poor post fracture performance could be linked to low bulk volume water (BVW). Oil-based re-fracture programs were designed and placed in both wells resulting in economic production. Log analysis concluded that formations with BVW%s of 3–3.5' or less to sandstone are better suited to an initial oil fracture stimulation.

Because the crossed-linked water stimulations sanded-off early with only a few tonne of proppant placed in the reservoir, two tonne of 100 mesh sand was added to the pads of the oil stimulations to help increase placement to 20 and 28 tonne, respectively. The increased propped fracture length and flowback of the oil load fluid combined to make the stimulations successful.

Company wide, the cost savings from water-based stimulations have provided a higher netback, increasing the number of new well candidates for fracture technology, thus increasing overall production and reserves. Work is currently underway to provide a more reliable log-analysisbased screening method for identifying which wells will definitely not accept a cost saving water fracture treatment, thus saving some re-fracture costs.

Introduction

Oil & gas companies in North America compete for the sale of produced oil with the rest of the world. Cost per barrel to produce is always the key issue. Companies try continually to reduce the costs of drilling, completion, and tie-ins, while using newer technology to increase production rates and reserves. Wells drilled today tend to be deeper as conventional oil and gas becomes harder to find, adding to drilling and completion costs. This paper will show only the cost reductions for completions, specifically how fracture stimulation costs have been reduced.

In general, refracturing of wells to enhance production has met with moderate success, mainly because many reservoir pressures have already been drawn down to free-gas conditions and a new frac produces under three-phase flow instead of two phases, essentially cutting oil production by a factor of 4 times due to relative permeability effects. Gas rates even as low as 5 e3m3/d affect oil production of less than 5 m3/d. Load fluid clean-up can also be a problem due to low energy flowbacks.

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