Reduction of Fluid Loss|
Two types of fluid loss to the formation can occur. These are seepage and lost circulation. The most common brine fluid loss is seepage. Seepage is the slow and steady flow of brine into the formation. Lost circulation, which does not occur often with brines, generally implies a fracture or breakdown of the formation, resulting in rapid fluid loss. The focus of this section is prevention of seepage losses to the producing formation.
Seepage loss occurs when the hydrostatic pressure head on the fluid is greater than the formation pressure. Seepage can range anywhere from near zero to as much as several barrels per hour. Seepage into the completion interval should be controlled for several reasons. Well pressure control is difficult when the wellbore fluid is seeping into the formation. In some cases, there are brine losses to one formation section and gas influx into the wellbore from another formation section. The influx of gas may be stopped by limiting the brine loss rate to the point where gas is not able to flow into the wellbore. Even though brines are designed to minimize formation damage, they are foreign to the formation. Therefore, uncontrolled brine penetration to the formation may disrupt the chemical balance and cause damage. One method of reducing excessive seepage loss is to decrease the pressure differential by reducing the density of the brine. Other solutions to seepage loss include raising the viscosity of the brine and adding sized bridging particles.
Fluid Loss Strategies
The most apparent method of fluid loss control is reducing density to lower the hydrostatic head against the formation pressure. However, well control and safety must be taken into consideration before reducing the fluid density.
The most common form of fluid loss control is to pump a viscous pill into the thief zone. This viscous pill consists of the CBF with the addition of polymer viscosifiers. The viscous fluid pill reduces the seepage rate. The rate of fluid influx to the formation depends on the pill viscosity and hydrostatic head exerted on the column of the brine. The two most common viscosifiers are:
- TETRAVis L Plus, prehydrated HEC (hydroxyethylcellulose); and
- BioPol L, prehydrated xanthan gum.
Viscous pills are the ideal first option to slow down the fluid loss rate and are applicable in formations with permeabilities less than one darcy and temperatures within the limits of the polymer selected. These pills are solid free and degrade with respect to temperature, losing fluid viscosity with time. Therefore, well flowback will remove the remains of the viscous fluid from the formation.
In cases of extremely high fluid loss, bridging solids may be used in combination with increased viscosity to stop the seepage. Usually two types of bridging solids are used. These are calcium carbonate (CaCO3) and sized common salt (NaCl). Calcium carbonate may be added to any viscous fluid. Sized salt pills must be saturated with sodium chloride to prevent dissolution of sized salt into the brine.
In order to select the proper sized bridging agent, the size of the pore holes to be bridged must be known. In general, particles will bridge an opening three to five times larger than the bridging particle diameter. Using this rule of thumb, a formation with an average pore size of 10 microns would require a bridging agent with particles averaging two to three microns in diameter. Proper bridging agents have a particle size distribution which allows a much tighter seal on the formation face, resulting in better fluid loss control. For example, the two to three micron bridging agent mentioned previously would most likely have a particle size distribution ranging from one micron to 20 or 30 microns. This material would have particles sufficiently large enough to bridge off the pore throats as well as particles small enough to form an impermeable cake. Bridging materials range in size up to fractions of an inch in order to handle the full range of fluid loss problems encountered in the field.
The fluid loss pill design must include planning for the cleanup of bridging solids after the completion process. When perforations are sealed or the open hole is bridged off, the removal of the bridging solids is not likely to be uniform. Even though a dissolving fluid is passed from the wellbore to the formation (acid, brine, or solvent washes) or from the formation to the wellbore (produced fluids), some perforations or sections of the formation face could remain sealed off, regardless of the type of bridging solids. If sections of the formation remained sealed at the wellbore, then the flow dynamics of the formation may be dramatically altered, and, as a consequence, production can be impaired.
The two methods for removing bridging materials are:
- CaCO3 spontaneous cleanup when the formation is allowed to flowback or by acid treatment with 15% hydrochloric acid, and
- salt removal by pressure differential during flowback, dissolution by formation water, or washover with undersaturated brine.