Abstract

The present study proposes, through the coupling of a linear filtration formulation (lab configuration) and a radial single phase formulation (wellbore vicinity), to predict fluid invasion depth of drilling fluid filtrate in the reservoir rock. Modeling is validated with linear and radial lab tests as well as with resistivity logs run in offshore wells from Campos basin. The proposed methodology is a requirement for optimum drilling fluid design to be used in the drilling of reservoir sections in both exploratory and development wells in Campos Basin, offshore Brazil.

Introduction

Minimizing fluid invasion is a major issue while drilling reservoir rocks. Large invasion may create several problems in sampling reservoir fluids in exploratory wells. Unreliable sampling may lead to wrong reservoir evaluation and, in critical cases, to wrong decisions concerning reservoir exploitability.

Besides, drilling fluid invasion may also provoke irreversible reservoir damage, reducing its initial and /or its long term productivity. Such problem can be critical in heavy oil reservoirs, where oil and filtrate interaction can generate stable emulsions. Invasion in light oil reservoir is less critical due to its good mobility properties (Ladva et al.1). Other critical scenario is the low permeability gas reservoirs where imbibition effects may result in deep invasion.

In order to avoid these problems, the drilling fluids industry spends a lot of effort providing non-invasive systems (Reid et al.2, Luo et al.3, among several others). A common practice in the industry is the addition of bridging agents, such as calcium carbonates in the drilling fluid composition. Such products would form a low permeability layer at the well walls which would control invasion. Several authors present fluid composition optimization studies for specific situations (Krilov et al4, among others).

Field Evidences of Drilling Fluid Invasion

Formation damage cause evaluation and diagnosis is a complex and very important topic since it may define effective or non-effective stimulation methods. Frequently it is difficult to define if drilling fluid invasion either was or was not a cause for a bad IP result.

Direct evaluation of filtrate invasion is also an important and complex issue. LWD and wireline logging are the most effective tools available. LWD resistivity responses establish the initial situation in the well while logs provide semi-quantitative invasion information regarding invasion at different distances from the well at the end of the drilling phase. MDT results indicate invasion when the sampled fluid contains filtrate. However, it may be imprecise to use such data to predict radial invasion due to permeability anisotropy effects. In this case filtrate could be migrating vertically from the reservoir, directly to the sampling chamber.

Field Examples

In order to illustrate fluid invasion evaluation and the impact of the nature of drilling fluid on the process, two wells drilled in adjacent areas (at an offshore Brazil exploration area) are considered. Well A was drilled with a cationic water based fluid while Well B was drilled with a synthetic based fluid. Main fluid properties, collected from rigsite data, are reported at Table 1. Besides standard filter paper tests, filtration experiments were also run through consolidated ceramic disks at the pressure differential which characterized the overbalanced operation (750 psi for Well A and 200 psi for Well B). Testing time was 3 hours and the results for Well A fluid are highlighted in Fig. 1. Results for synthetic fluid indicated minimum filtration (less than 2 cc) after 3 hours.

Fig. 2 shows, for Well A, the porosity (continuous black line), density (continuous red line) at the right side and the group of 5 resistivity logs at the left side. The dotted yellow color represents the reservoir sections in the log. The dark blue resistivity log (extreme right of the figure), representing the well bore vicinity, denotes the baseline for the water base drilling fluid filtrate. This line shows lower resistivity than the other curves which represent the lower salinity formation water at the shale intervals. In the reservoir the other four resistivity logs tend to approach the base line indicating that the high salinity filtrate has deeply invaded the reservoir.

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