Excessive loss of high-density brines into the formation has always been a major concern during completion operations, since it leads to formation damage and well control issues. The problem becomes more complex at high temperatures and when the treatment involves running gravel-pack assemblies and downhole sand screens.
Typically, the fluidloss-control pills are composed of very high concentrations of crosslinked polymers with or without bridging particulates. The sealing mechanism of these pills is a combination of viscosity, solids bridging, and polymer filter-cake buildup on the porous rock. Due to the instability of polymers at high bottomhole temperatures, incompatibility with some divalent brines, and the necessity to cleanup with acid, a new solids-free lost-circulation pill that is stable for prolonged periods at high temperatures was developed.
This paper introduces the development and the first field application of a new solids-free non-damaging viscoelastic surfactant-based fluid-loss pill (VES-PILL). The single-additive system forms a "gel" with most completion brines currently used in well operations. Laboratory data demonstrate that this pill could be used up to 375°F. The "gel" structure of this system sustains viscosity high enough to effectively control or stop brine-loss, while maintaining a safe differential pressure into the formation.
Several "frac and pack" completions were performed for the first time in Saudi Arabia in the Pre-Khuff sweet-gas zones. The VES-PILL was used to prevent losses after perforating, fracturing and gravel-pack operations. The pill was used in the field up to 310°F, and brine-loss was effectively controlled for more than 3 days. The effectiveness of the pill was also demonstrated by a five-fold increase in surface pumping pressure during placement. The wells were produced at rates exceeding expectations without further remediation to cleanup the fluid-loss pill.
Background
Fluid-loss control is very important in successful well completion operations. Loss of completion and workover fluids is unacceptable due to economic (expensive heavy brines), technical or safety reasons (formation damage, hole collapse and well control issues).1 Loss of dense brines into the productive zones is highly damaging, especially to high permeability formations. It is very difficult to unload heavy brines once losses have occurred. Because of the high-density of brines used, stratification tends to further inhibit its removal.2 Calcium and zinc bromide brines can form stable acid insoluble complexes when reacted with some formation brines.3 Hence, the most effective means of preventing the formation damage is to limit completion brine losses into the formation by either chemical or mechanical means.
It is best to avoid the use of fluid-loss control pills by incorporating mechanical fluid-loss control devices into the completion string whenever possible.4 However, in the absence or failure of such devices, or in situations where they cannot be used, chemical fluid-loss pills are required. The use of a pill is normally required before and after sand control treatments and after perforating. In these treatments, the pill is spotted into the perforations or against the sand control screens. In addition, fluid-loss control pills are required in several workover operations that need temporary zonal isolation. There are several reviews on the use of different types of fluid-loss pills5–7 and guidelines on the selection of the pill.4,8
A variety of fluid-loss control pills have been used in the industry, such as foams3, oil-soluble resins3, fibers,5,9 acid soluble particulates,3 graded salt slurries,10 high concentrated linear11 and crosslinked1,6,1,13 non-biopolymers14 and bio-polymers.10,15 The polymer systems are very effective in fluid-loss control as long as the temperature limit of the specific polymer is not exceeded.
One of the important features of any fluid-loss pill is its ability to maintain viscosity under bottom-hole conditions (especially high temperature). The viscosity reduction of gel at high temperatures is either due to the degradation of polymer or reduced molecular interations.16 The viscosity will not be regained on cooling if there is molecular degradation at high temperature.
