Abstract

Drilling departments focus the design of drilling fluid on volume and cost minimization. For the cementing operations, special care is given to reducing additive and spacer/preflush usage.

The production department has to deal with maximizing production. Those objectives are very often not complementary and even sometimes opposite. The drilling department will therefore be successful if the well is drilled and cemented at minimal costs without any major problems. However when the well is handed over to the production department, any fluids or solids invasion will affect production dramatically and the same well could be seen as a "failure". This paper tries to demonstrate that minimizing formation damage could be used as primary objectives for drilling and cementing fluid designs and still maintain or even lower the well cost while maximizing the well potential. A hypothetical horizontal well is taken as an example. Fluid designs as well as potential remedial operations are taken into account. Laboratory tests are used for formation damage evaluation as well as some software simulations to evaluate skin damage.

Introduction

Formation damage has to be considered as any process that impairs the permeability of reservoir formations such that production or injectivity is decreased. For many years the drilling industry focused on practices which gave high rates of penetration and minimum wellbore problems. The cementing industry focused most of the time on designing slurries which will not bridge up or prematurely set within the casing. As a consequence, drilling and cementing fluids were often formulated to drill and cement the well cheaply and quickly with little thought of the impact on well productivity. However, other considerations such as the increase in small and marginal field development and the increasing number of wells being completed without casing and perforation should stress the importance of understanding and controlling formation damage.

However other reasons exist which should increase the industry awareness of formation damage:

  • The increase in small and marginal field developments. This often means drilling only a small number of wells which must produce to their full potential.

  • Stimulation treatment could cure formation damage. However prevention of damage is better than cure and could make the well cheaper.

  • An increased number of wells are completed without casing and perforation.

In these completions, near-wellbore damage is not bypassed by perforating tunnels. Hence formation damage has to be minimized. These considerations explain the importance that should be given to the understanding and control of formation damage when designing drilling and cementing fluids.

On the other hand, avoiding formation damage is not possible. Non-damaging fluids do not exist. The question is to find out what levels of formation are acceptable taking into account parameters such as number of wells, type of formation, type of completions and so on. In this paper, the mechanisms of formation damage by drilling and cementing fluids are reviewed. The formulation of low-damage well construction fluids is also discussed. Comparisons are made between "conventional" systems and low damaging ones from a cost and productivity point of view.

Experimental

Two different formation damage tests are performed on core samples (1 inch diameter, 2 inches long). The first test is carried with the fluid filtrate while the second test is conducted with the whole fluid, mud or cement. The following procedures were as follows:

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