RIASSUNTO
ABSTRACT
One of the most important uses for polymers in drilling fluids is shale stabilization. The shale stabilizing mechanism is not very well understood, but many think it results from hydrated polymer molecules interacting with water and shale surfaces by a process of adsorption. Adsorption is a complex surface phenomenon. Measurement of solution viscosity may not be adequate to quantify the adsorption characteristics of polymers. This study focuses on the static adsorption of a partially hydrolyzed polyacrylamide polymer (PHPA) on formation solids. The study also investigates various factors affecting adsorption of polymers. The adsorption was quantified by adsorption density, which is the amount of polymer adsorbed per unit surface area of solid particles. Application of laboratory findings to field situations is also discussed.
INTRODUCTION
Water based drilling fluids containing PHPA (partially hydrolyzed polyacrylamides) are used offshore worldwide, particularly in deep water drilling. Some of the problems of running PHPA mud from field experiences are the following:
Separation of water from mud after addition of PHPA polymers.
Mud loss from solid control equipment, particularly at shale shakers.
Fast build-up of drill solids which causes thickening of mud and reduces mud pumpability.
Slow dispersion of polymers which creates transient, high apparent viscosity of the drilling fluid.
Loss of polymer over the shakers because of polymer fish-eyes.
Detectable ammonia smell at the mud pit where an excess of caustic or cement contamination can cause extensive hydrolysis of PHPA polymers.
Lack of test equipment and methodology, which can be used to monitor PHPA concentration in the mud at flow line and suction pit.
Many papers suggest the cause of these problems could be due to one of the following:
Flocculation occurring in the suction pit when additing PHPA polymers.
High concentration of PHPA in the returning mud, thus reducing efficiency of shale shaker to handle low gravity solids.
Insufficient agitation hampering dispersion of polymers.
Improper pH control and/or cement contamination of drilling mud, causing hydrolysis of PHPA polymers and creating ammonium hydroxide.
Furthermore, many publications emphasize performance evaluation of PHPA drilling fluids. Authors rarely disagree that the primary benefit of using the PHPA system is to obtain a gauge or near-gauge hole and to achieve borehole stability. In view of all attempts to render PHPA mud systems effective and economical, there is a need to gain more understanding of how PHPA polymers inhibit shale hydration.