|Publisher||Society of Petroleum Engineers||Language||English|
|Content Type||Conference Paper|
|Title||Drilling Through Highly Fractured Formations: A Problem, a Model, and a Cure|
|Authors||Santarelli, F.J., Dardeau, Claude, Zurdo, Christian, Elf Aquitaine|
SPE Annual Technical Conference and Exhibition, 4-7 October 1992, Washington, D.C.
|Copyright||Copyright 1992, Society of Petroleum Engineers Inc.|
It is usually accepted that wellbore instabilities are caused by either or both an excessive stress concentration at the borehole wall and the chemical reactivity of the formation. Assuming good hole cleaning, common cures for such instabilities are therefore mud density increase and/or change of the mud system. However, even though these methods have proved highly successful when drilling through intact formations, the same may not stand when it comes to highly fractured rocks. The purpose of this paper is to describe the studies performed in order to allow the safe drilling of a particularly troublesome, highly fractured formation which had led previously to several successive side tracks.
The efficiency of the different attempts at solving the problem are analysed in the light of the various side tracks. The core taken during one of these side tracks was analysed, showed that the formation was highly fractured and chemically inert, and provided many parameters for the modelling of the problem which was performed by a discrete element programme. Further modelling was performed on the mud hydraulics. Both theoretical and field data show that density increase has, in such a case, a negative role on borehole stability while filtrate reduction and mud rheology enhancement are highly positive.
The drilling of wells under ever more difficult conditions such has greater depth, longer phase duration, high pressure and high temperature environment, etc, has revealed a crucial need for better understanding of borehole stability under various loading conditions. The main thrust of recent research in this area has used the continuous medium concept (continuum mechanics), as it was believed that the average length between natural fractures in rocks was always large enough when compared to common wellbore diameters. The fundamental approach of such studies consists in applying linear isotropic elastic equations in order to compute the stress redistributions around the well and in using various criteria for either compressive or tensile failure in order to compute a collapse pressure or a fracturing pressure for the well. The various loading conditions affecting wells during the drilling phase such as thermal stresses, mud hydraulic support efficiency, pore pressure, etc, were analysed in both their steady state and transient aspects. Various improvements have been made to the original approach in terms of rock rheology, failure criteria, etc, which are summarized and evaluated in different recent reviews.
However, recent field observations have shown that despite their relatively small diameters, wellbores can also be affected by the presence of natural fractures in the rock mass. For instance, it was reported how, after a long series of operations, a deep gas well was lost by the pressurization of a fault which led to the complete collapse of the casing. Further evidence of movements along discontinuities at the borehole wall can be obtained by using borehole imaging techniques.
The aim of this paper is first to present a field case when drilling through heavily fractured formations led to important borehole instability problems. The various mud parameters used to try to solve the problems will first be introduced. In a second part, a core which was taken during one of the various side tracks will be described along with the various laboratory analyses performed on it. After, the core observation, it was decided to use a commercial Discrete Element Model (DEM) named UDEC in order to understand the mechanisms which had led to the instabilities experienced in the field ; this will be presented next. The use of an inhouse developed model (LORAH) which studies the mud circulation in the well will be presented and it will be shown how the mud parameters were adjusted to minimize the erosion of the borehole wall. The main results of the study will finally be discussed and it will be shown how they were extended successfully to several other field cases.
|File Size||587 KB||10|