Optimizing filtrate design to minimize in-situ and wellbore damage to water-wet reservoirs during drill-in

Abstract

A prominent area of concern related to drill-in and completion operations is damage caused by polymer and/or brine filtrate invasion into the producing formation. It is common knowledge within the petroleum industry that, in overbalanced conditions, filtrate invasion into the pore spaces of a reservoir is continuous as long as the drill-in fluid and wellbore are in contact. It is also commonly known that certain polymers and modified starch additives tend to adsorb or to be retained, and sometimes plug, the water-wet linings of reservoir pores. Once this occurs, flow paths are restricted and formation permeability is impaired. Depending on the compatibility of the aqueous filtrate with the reservoir fluids, strong water/oil emulsions may be formed and also plug reservoir pores.

This paper presents credible laboratory data demonstrating the benefits of a special surfactant molecule engineered to pass through a drill-in fluid filter cake along with the other filtrate components to maximize wellbore productivity.

Introduction

There are numerous factors influencing the degree of damage to producing formations during the reservoir drill-in phase with water-based fluids. The drilling fluids industry has made significant progress in fluid designs to minimize reservoir damage by customizing drill-in fluid formulations based on reservoir characteristics.

It has been extensively shown that water-based fluids (WBM) do not affect or modify reservoir wettability but significantly reduce reservoir permeability. Thus, one of the main formation damage mechanisms that still needs to be prevented is waterblock. This effect is due to the retention and adsorption phenomena of polymeric additives which are not retained in the filter cake and can deeply invade the reservoir.

The objectives of the study presented here are to design and select a special surfactant molecule engineered to pass through a drill-in fluid filter cake along with the other filtrate components to maximize wellbore productivity and prevent

formation damage with water-based fluids i.e. completion or drill-in fluids (DIF).

The surfactant discussed in this paper is identified as Radiagreen RA, for reservoir activator. Prior to its use in the field, RA was tested for toxicity and biodegradability and has been assigned a GOLD rating using the CHARM classification model.

State of the art

The ability of surfactants to affect mineral surfaces is well known and has been widely studied. When used to drill through the producing formation, it has been observed that some of the surfactants used in invert emulsion drilling fluids contribute to a reduction in the production potential of a reservoir through a variety of formation damage mechanisms.

A set-up with a sandpack permeameter and invert emulsion reservoir drill-in fluids were used for the experiment. It should be noted that the relative permeability values given for each of the fluids tested increases the oil permeability in the rock matrix through a reduction in the trapped water layer. The relative permeability is increased after exposure to invert emulsion fluids if the brine emulsifier additive does not tie up the connate water and create an emulsion block.

Also to be noted is that one of the standard procedures for return permeability testing, calls for an initial oil flow in the production direction before fluid-off. Flow is continued until the state of maximum irreducible water is achieved.

This brings us to surfactant additions for water-based drill-in fluids. Can the permeability of a water-wet formation be increased in the near wellbore zone by reducing the volume of trapped water when contacted by invaded filtrate containing a specialized additive?

Laboratory Experiments

Invasion in porous media: For all the work performed in porous media, Clashach sandstone cores were used in the experiments. The cores were saturated with a brine and brought to residual water saturation (S wi) by flowing with kerosene in the opposite direction to filtration. The kerosene permeability at S wi was measured at different flow rates and taken as the undamaged initial permeabil ity (Ko Swi).

After the injection of a certain number of pore volumes of filtrate while we determine the volume of oil recovered, we simulate oil production by flowing oil in the core in the opposite direction and determine the residual oil permeability. Initial and final oil and water saturations are determined with a precise mass balance of water and oil recovered if possible.

Static and Dynamic Filtration:
Static and dynamic filtration tests were performed in an API filtration cell or in a dynamic filter press.

Emulsion Block Risks:
evaluation by using classical bottle tests. These tests were performed using DIF or completion fluid filtrate and crude oil or s ynthetic and crude oil. The potential for emulsion formation between formation fluid and fluid filtrate was also evaluated. Different ratios of crude/fluid filtrate were prepared with and without the 1g/L RA surfactant.

Results

Effect of Radiagreen RA on water/oil contact:
The Radiagreen RA additive is effective (=decreases surface tension) at low concentration, even far from the CMC of this molecule (around 100 ppm). Ageing does not affect the measurement.

Compatibility of Radiagreen RA with DIF formulations:
The additive does not impact the filtration and rheological properties of water-based fluids even after ageing. Torque measurements do not point out any interaction between the lubricant and Radiagreen RA additive.

Effective concentration of Radiagreen RA within the filtrate:

Classical static filtration test was performed in API cell at room temperature: the Radiagreen RA surfactant does not have any effect on filtration, furthermore the Radiagreen RA is passing through the filter cake.

Invasion in porous media:
The oil permeability is enhanced when the imbibition step occurs in the presence of Radiagreen RA and the water saturation is strongly decreased.

The presence of Radiagreen RA prevents the PAM plugging effect and limits the amount of trapped water in the near wellbore.

Return Permeability:
The results are consistent with results obtained previously. The presence of Radiagreen RA additive in the filtrate promotes the ease oil production and suggests that higher flow rates are possible.

The presence of Radiagreen RA in a drill-in fluid formulation containing solids is similar than previously observed.

Emulsion Block Risks: T he presence of the Radiagreen RA additive in the fluid filtrate induced emulsion breaking even in the presence of heavy crude oil. Heavy crude oil is recognized to promote very stable emulsions in porous media.

Conclusions

• A reduction of surface and interfacial tension is obtained when the Radiagreen RA surfactant is added to water-based drill-in fluids or completion brine.
• The addition of the Radiagreen RA surfactant to water-based drill-in fluids can prevent the formaton of in-situ water/oil emulsions and reduce the risk of water blockage.
• Polymer-based drill-in fluids, when evaluated without the Radiagreen RA surfactant, promote water-blocks in the pore spaces by increasing the water saturation levels. In addition, the hydrated polymer may further adsorb onto the water-wet rock surface and partially plug the reservoir pores.
• Polymer based drill-in fluids, when evaluated with the Radiagreen RA surfactant, minimize the formation of water-blocks by reducing the water saturation levels and prevent the binding of the hydrated polymer to the rock's surface.
• The use of the Radiagreen RA surfactant in polymer-based drill-in fluids results in an increase in relative permeability when it is compared to the same systems without the Radiagreen RA surfactant .