Fluid loss additives

Comparative tables of fluid loss additives can be found in the internet [1]. Fluid loss additives are also called filtrate-reducing agents. Fluid losses may occur when the fluid comes in contact with a porous formation. This is relevant for drilling and completion fluids, fracturing fluids, and cement slurries.

The extent of fluid loss is dependent on the porosity and thus the permeability of the formation and may reach approximately 10 t/h. Because the fluids used in petroleum technology are in some cases quite expensive, an extensive fluid loss may not be tolerable. Of course there are also environmental reasons to prevent fluid loss.

2.1 Mechanism of action of fluid loss agents

Reduced fluid loss is achieved by plugging a porous rock in some way. The basic mechanisms are shown in Table 2.1.

2.1.1 Pore size measurement by nanoparticles

The development of temperature sensitive and pressure sensitive nanosensors will enable in situ measurements within the reservoir. The parameters involved in the mobility of nanoparticles through porous and fractured media were investigated [2]. These parameters were particle size or size distribution, shape, and surface charge or affinity to rock materials.

It was found that spherically shaped nanoparticles of a certain size and surface charge are compatible with that expected in formation rock are most likely to be transported successfully, without being trapped because of physical straining, chemical, or electrostatic effects. Tin-bismuth nanoparticles of 200 nm and smaller can be transported through Berea sandstone. Larger particles were trapped at the inlet of the core, indicating that there was an optimum particle-size range. On the other hand, the entrapment of silver nanowires occurs primarily because of their shape [2].

The investigation of the flow mechanism of nanoparticles through a naturally fractured greywacke core was conducted by injecting fluorescent silica microspheres. Silica microspheres of different sizes (smaller than the fracture opening) could be transported through the fracture.

Thus, it was demonstrated that by using microspheres it is possible to estimate fracture aperture by injecting a polydisperse microsphere sample [2].

2.1.2 Action of macroscopic particles

A monograph concerning the mechanism of invasion of particles into the formation is given by Chin [3].

One of the basic mechanisms in fluid loss prevention is shown in Figure 2.1. The fluid contains suspended particles. These particles move with the lateral flow out of the drill hole into the porous formation. The porous formation acts like a sieve for the suspended particles. The particles therefore will be captured near the surface and accumulated as a filter cake.

The hydrodynamic forces acting on the suspended colloids determine the rate of cake buildup and therefore the fluid loss rate. A simple model has been proposed in the literature that predicts a power law relationship between the filtration rate and the shear stress at the cake surface [4].

The model shows that the cake formed will be inhomogeneous with smaller particles being deposited as the filtration proceeds. An equilibrium cake thickness is achieved when no particles small enough to be deposited are available in the suspension. The cake thickness as a function of time can be computed from the model.

For a given suspension rheology and flow rate there is a critical permeability of the filter, below which no cake will be formed. The model also suggests that the equilibrium cake thickness can be precisely controlled by an appropriate choice of suspension flow rate and filter permeability.

2.1.3 Action of cement fluid loss additives

Two stages are considered with respect to the fluid loss behavior of a cement slurry [5]:

1. a dynamic stage corresponding to placement and

2. a static stage, awaiting the setting of the cement.

During the first period, the slurry flow is eroding the filter cake as it is growing; thus a steady state, in which the filtration occurs through a cake of constant thickness, is rapidly reached. At the same time, because the slurry is losing water but no solid particles, its density is increasing in line with the fluid loss rate.

During the second period, the cake grows because of the absence of flow. It may grow to a point at which it locally but completely fills the annulus: Bridging takes place and the hydrostatic pressure is no longer transmitted to the deeper zones. From the typical mud cake resistance it can be estimated that under both dynamic and static conditions, the fluid loss could require reduction to an American Petroleum Institute (API) value lower than what is generally considered a fair control of fluid loss.

2.1.4 Testing of fluid loss additives

Fluid loss prevention is a key performance attribute of drilling fluids. For water-based drilling fluids, significant loss of water or fluid from the drilling fluid into the formation can cause irreversible change in the drilling fluid properties, such as density and rheology occasioning instability of the borehole. Fluid loss control is measured in the laboratory according to a standard procedure for testing drilling fluids [6].

Predictions on the effectiveness of a fluid loss additive formulation can be made on a laboratory scale by characterizing the properties of the filter cake formed by appropriate experiments. Most of the fluids containing fluid loss additives are thixotropic.

Therefore, the apparent viscosity will change when a shear stress in a vertical direction is applied, as is very normal in a circulating drilling fluid. For this reason, the results from static filtering experiments are expected to be different in comparison with dynamic experiments.

Static fluid loss measurements, provide inadequate results for comparing fracturing fluid materials or for understanding the complex mechanisms of viscous fluid invasion, filter cake formation, and filter cake erosion [7]. On the other hand, dynamic fluid loss studies have inadequately addressed the development of proper laboratory methods, which has led to erroneous and conflicting results.

Results from a large-scale, high-temperature, high-pressure simulator were compared with laboratory data, and significant differences in spurt loss values were found [8].

Static experiments with pistonlike filtering can be reliable, however, to obtain information on the fluid loss behavior in certain stages of a cementation process, in particular when the slurry is at rest.

2.1.5 Formation damage

The damage of the formation resulting from the use of a filtration loss agent can be a serious problem for certain fields of application. Providing effective fluid loss control without damaging formation permeability in completion operations has been a prime requirement for an ideal fluid loss control pill.

Filter cakes are hard to remove and thus can cause considerable formation damage. Cakes with very low permeability can be broken up by reverse flow. No high-pressure spike occurs during the removal of the filter cake.

Typically, a high-pressure spike indicates damage to the formation and wellbore surface because damage typically reduces the overall permeability of the formation. Often formation damage results from the incomplete back-production of viscous, fluid loss control pills, but there may be other reasons.

2.1.6 Reversible gels

Another mechanism for fluid loss prevention is caused by other additives, which are able to form gels on a molecular mechanism.

2.1.7 Bacteria

Instead of using polymers, the addition of bacteria cultures, which may form natural polymers and could then prevent fluid loss, has been...