Abstract

In most of its deepwater Campos Basin oilfields, Petrobras' strategy has been to develop the water flooding systems using horizontal gravel packed wells. Both drilling and gravel pack operations are only possible through the presence of a filter cake, formed on the formation face during the drilling process. Nevertheless, that film becomes an impediment to injection and must be removed to obtain good injectivity.

Special tools were developed to perform acidizing jobs, in one trip, just after the gravel packing operation. However, there were few alternatives to correct any eventual acidizing lack of performance, besides using coiled tubing and straddle packers, something that usually takes too much time.

Recently, a pulse-rotating jet tool system was applied in a well that could not be treated after the gravel pack operation. The treatment, which is simpler and faster than those carried out with straddle packers, was performed in two stages: the first, using just HCl, and the last one, using mud acid.

The subsequent formation test performed after the HCl treatment showed a high skin value and an unacceptable injectivity index, due the need to keep the reservoir pressure above the bubble point. Even so, the test data in association with reservoir simulation, according with the field geology, showed that there was fluid injection throughout the entire horizontal section. Therefore, it was decided to perform a mud acid treatment, again, using the pulse-rotating jet tool system.

This time, a second formation test indicated an excellent injectivity index, confirming the pulse-rotating jet tool system effectiveness and creating perspectives for the application of this technique in future operations.

This paper will describe the technique, job details and the results obtained for this particular operation, which may be applied in other similar offshore environments.

Introduction

Horizontal open hole gravel packs have been used as Petrobras main strategy for the development of deepwater fields. Although the producers have usually presented very good results since the beginning of its utilization, the water injectors have presented a substantially more complex challenge, once the damage caused during the formation drilling, despite the care taken during this phase and all fluid control, have always produced a big reduction in the injectivity.

Hypotheses were raised to explain the observed damage and the behavior differences between producers and injectors, based on the drill in fluid composition, drilled rock / layers composition and, finally, the filter cake composition and the behavior of its components.

According to those arguments, two different approaches were developed:

- Chemical treatments to remove damage, nowadays entirely focused in the filter cake;

- Special dedicated tools, to treat the injectors just after the gravel packing operation.

Normally, the association of the developed tools and treatments have produced great results, but, in some few cases, the acidizing job had to be postponed, or even did not present the desired effect, making it necessary to repeat the treatment. In such situations, the use of dedicated gravel packing and acidizing tools is always very complex and expensive, so, coiled tubing and straddle packers were normally used.

Recently, however, a pulse rotating jetting tool was alternatively applied, and demonstrated to be very fast and efficient to place the acid treatment, as well as to remove formation damage, which brought great perspectives to other treatments that will have to be performed in non conventional conditions, or even to those who will have to be repeated, due some unexpected response.

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