Abstract

In recent years, frac-pack completions have been proven to be a means for obtaining highly efficient completions in high-permeability, unconsolidated formations. The success of this completion technique has been attested to by it's moving into a strong lead as the most common sand control completion. Along with its wide-spread acceptance, care must be exercised to insure that we, as an industry, are not becoming too complacent concerning proper procedures.

For example, it is becoming increasingly common to "cut corners" during frac-pack execution, and eliminate pre-job field evaluations of mini-frac tests. These short cuts are typically taken in well-developed areas where it is thought little can be learned from the additional data, and the time savings are seen as significant. However, the difference in conditions as well as specific fluid characteristics from well to well, can cause less than optimal treatments being pumped if appropriate pretreatment evaluations are not performed.

This paper contains two field cases where the frac pack treatments resulted in exceptionally good production rates, low sand-face draw-downs and complete sand control. These were not straight-forward, hit and run type exercises, and both required extensive effort and forethought in terms of design, applied technology and operations. This suggests that the time spent in aligning the treatment scope with a particular reservoir needs during the execution phase of the Frac Pack completion played a significant role in the success of these completions. Therefore, when considering the use of pretreatment evaluation procedures the cost should not be viewed as "thousands of additional dollars spent" but indeed as "millions of dollars not left on the table" through optimized productivity.

Introduction

There are many different estimates as to when frac-packing was introduced as a viable completion alternative for unconsolidated formations. While it is often claimed that the practice of combining a hydraulically induced fracture with a gravel pack started in Venezuela during the 1980's1, other reports indicate that the term was actually used as early as the late 1950's by Shell in Germany2. It is widely recognized that frac-packing as it is known today has come to maturity since the early 1990's3. Regardless of when it began, frac-packing is indeed widely recognized as a premier completion technique.

During the past 10 to 15 years the benefits of frac-packing over conventional gravel packing have become well known. It was commonly stated in the literature of the early 1990's that conventional gravel packs (gravel placed below fracture pressure) could easily exhibit skin values exceeding +20. The cause of this poor well performance was identified as the effect of the near wellbore damaged zone. This formation damage was considered to be a result of the combined effects of drilling fluid filtrate, cement filtrate, perforating damage, and the damage associated with the gravel pack carrier fluid. At first it was hoped that this damage could be removed through acidizing. However, it was soon demonstrated that it is quite difficult to remove all of the damage with acid alone. Therefore, a solution was sought that would provide an unobstructed flowpath through the damaged zone. The idea was that a hydraulically induced fracture filled with high permeability proppant could provide the solution.

It was also recognized that a standard hydraulic fracture would not be suited for a damage bypass treatment in a high-permeability, unconsolidated formation. First, it must always be remembered that sand control will be needed. Second, due to the high-permeability of the unconsolidated formations, it is understood that a wide fracture is needed. To obtain this wide fracture a process known as tip-screenout (TSO) fracturing is utilized. To successfully implement a TSO fracture treatment it is critical that fluid volumes, and pump rates be balanced with the fluidloss rate to the formation. In addition, since wide fractures tend to have unstable proppant packs (leading to potentially excessive proppant flowback - resulting in a less productive narrow fracture), some means to retain the proppant within the fracture was also needed. This need, as well as the need for sand control led to a frac pack combing a TSO hydraulic fracture with a gravel pack. However, because of this combined approach - it is necessary that proper field techniques be used to optimize both the fracture treatment as well as the gravel pack.

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