Abstract

It has long been postulated that placing acid into a well through coiled tubing provides for better stimulation treatments than those achieved by simply bullheading. Common sense suggests that having the stimulation fluid enter the well via a dedicated conduit, adjacent to all points along the length of a treatment interval, will produce more uniform stimulation of that interval. However, the opposing argument is that, irrespective of the delivery method, all fluid follows the path of least resistance and will, therefore, flow into the areas of highest permeability, the ones generally least in need of stimulation. Diversion techniques, employed to improve treatment distribution, can often be applied to regular bullhead treatments as easily or even more easily than they can with coiled tubing. This is not necessarily so when pumping stimulation fluids through SSD's or through the ESP itself.

Controlled tests to compare bullheaded and coiled tubing delivered treatments are difficult, as no two wells are identical. This paper evaluates 19 wells to provide statistical evidence of which delivery system does generally provide for the best end results. The paper suggests what the underlying reasons for the spread of results might be and suggests criteria as to when coil is, and is not, the best probable choice. It also discusses innovative methods to improve treatment distribution in wells, thereby maximizing stimulation efficiency.

Introduction

The Chinese National Offshore Oil Corporation (CNOOC), operates a number of fields offshore in the Java Sea, located in a block to the southeast of the island of Sumatra (see Figure 1). CNOOC, together with previous operators of this block (Maxus, YPF and Repsol) have for many years been using acid stimulation to enhance the production from their wells. Many different types of treatments have been performed, and many different methods for placing the acid have been used.

The wells operated by the NBU (North Business Unit) of CNOOC tend to be very high water cut (averaging 96%), relatively high permeability sandstones (Talang Akar formation). All sandstone wells are completed with screens and some kind of sand control, usually a gravel pack. All wells are produced using electric submersible pumps (ESP's) for artificial lift, as reservoir pressures tend to be low.

As 90% of CNOOC's producing intervals have a sand control completion, the predominant form of stimulation is matrix acidizing, using both HF- (hydrofluoric acid) and HCl- (hydrochloric acid) based systems. CNOOC and its predecessors have a significant history of stimulation successes1–3. The case histories presented, however, deal only with HF-based acid systems.

As every well is producing via an ESP, there is a major decision to be made when selecting and proposing a well as a candidate for acid stimulation. Can a workover be justified? Or, alternatively, is it nearly as effective to bullhead the treatments down the existing production string?

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