Abstract

Complex well configurations with long openhole sections are often left unprotected for a long time under various conditions. Oil-based and synthetic-based drilling fluids are very well suited for drilling these long sections down to reservoir formations, providing high rates of penetration, excellent lubrication properties, and wellbore stability. The enhancement in wellbore stability and lubricity as well as reduction in differential sticking, mud losses, and formation damage can be directly influenced by fluid-loss control and filtercake properties. As an example, formation damage is largely caused by solid invasion occurring during the first step of filtration, perhaps adversely affecting the porous media permeability/stability, and consequently, reservoir productivity. Both good fluid-loss control and minimum formation damage are required for drilling and completion fluids.

A synthetic polymer, based on a unique microgel structure, has been designed to provide excellent fluid-loss control and low formation damage in OBM/SBM. Optimization of polymer morphology and fluid solubility gives a very versatile and cost-effective fluid-loss additive.

The unique microgel structure allows the polymer particles to swell and stack up, forming a thin and efficient filter cake. The ability of the microgel particles to deform under shear stress allows them to be back-produced from the formation more effectively than non-deformable additives, thus reducing risks of formation damage in the pay zone. The polymer also may contribute to the rheology of the mud by providing low shear viscosity under demanding conditions.

This paper reviews the effectiveness of the organosoluble polymer by presenting evidence of low formation damage illustrated by good return permeability data from both field and laboratory applications. Finally, data will be presented on new developments based on similar chemistry, allowing applications into more extreme conditions of temperature.

Introduction

Owing to their excellent lubricity, high rate of penetration and outstanding thermal stability, synthetic-based mud (SBM) and oil-based mud (OBM) systems are often used to drill difficult wells, such as long sections of high angle and/or HTHP wells. However, the risks of having mud loss or invasion into reservoir rocks thus hampering production may become elevated because of long exposure time with the producing zone under overbalanced conditions.

Formation damage is largely caused by solids invasion that occurs during the initial stages of filtercake establishment when an internal filtercake is formed. The formation of an impermeable external filtercake is considered essential for minimizing formation damage, and this should take place as soon as the reservoir rock is penetrated. Once the filter cake is established, the filtration process is controlled by the cake itself rather than by the rock.

Different additives may be used in OBM/SBM formulations to enhance cake properties and minimize formation damage. These additives may be either dispersed or solubilized in the oil phase.1,2 A new class of synthetic polymers that functions as fluid-loss control and bridging agent is described in this paper. The synthetic polymer represents a range of organosoluble microgels based on a unique microstructure design. The polymer's morphology, solubility, and crosslink density have been optimized to enhance fluid-loss-control performance while maintaining cost effectiveness. The performance of the synthetic polymer in terms of fluid-loss control and formation damage is compared with traditional additives including gilsonite and amine-treated lignite (ATL).

Filtration Mechanisms and Characterizations

The drilling fluids filtration process is composed of two distinct stages. The first stage (also known as spurt) corresponds to the inception of filtration. It is characterized by a rapid invasion of the porous media by the drilling fluid. During this period, the solids present in the fluid invade the rock. An internal cake is built in the first few centimeters of the invaded formation and the invasion rate will decrease rapidly with good bridging. It has been previously shown that when no interactions exist between the filtrate and the reservoir fluids, the initial spurt loss is the damaging phase and that further fluid loss of the oil filtrate does not change the level of damage imposed by the drilling fluid.3

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