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Abstract

Experience has shown that adding suitable particles to the drilling fluid can significantly improve the formation strength. This is especially beneficial when drilling wells with a narrow operational window as is typically the case when drilling depleted reservoirs.  Successful operations have employed several types of particles, including graphite and calcium carbonate.  For this kind of approach to work, it is important to establish an optimal particle composition in the drilling fluid. In a recent paper, SPE 107574, the possibility for continuous monitoring of the particle size distribution (PSD) and particle content in the drilling fluid, was demonstrated.  This present work shows how these findings have allowed deploying a unique technique for running of solids control equipment in a North Sea offshore field, providing optimal particle concentration and size distribution when drilling a significantly depleted reservoir.  We describe in detail an HTHP offshore operation where coarse shaker screens were used to allow relatively large particles to re-enter the well during circulation.  These particles act just like particles deliberately added to the drilling fluid in order to enhance formation strength.  The monitoring equipment allowed for close control of the PSD of the drilling fluid flowing into and out of the well during drilling.  The particle size distribution is compared to that obtained with conventional additives previously used to achieve formation strength enhancement.

Introduction

Drilling through depleted reservoirs is a challenge since the pressure difference between the fracturing pressure and the pore pressure becomes small; or even sometimes negative.  During the last decade a technique has been suggested that increase the fracture strength towards the original strength, and, in some cases the new fracture strength has exceeded the original strength.  This method is based on fracturing the borehole wall with small fractures and then fill these with impermeable particles to stop further propagation of the fractures at the same time as the fractures remain increasing the formation strength of the remaining portion of the borehole wall.  The concept of increasing the formation strength while curing lost circulation has been discussed by Messenger¹ and Morita et al.².  Fuh et al.³ suggested using this method while drilling to prevent lost circulation.  A suggestion of the selection of material and the theoretical treatment were later refined by others^4-10.

Theoretical analysis and all analysis of field experience on formation strengthening conclude that it is necessary to optimize the particle size distribution of the added solids.  The fracture must be sealed by a non-permeable easily plugging material.  The plugging is caused by arching or gel formation or a combination of these two items.  Arching in pipes and conical sections has been a large subject for research the last century^11,12,13, and the results are included in most textbooks on soil -or powder mechanics¹⁴.  Although this subject is well established theoretically, there is still a need for experimentally optimizing the particle size distribution and particle content.  Furthermore, for drilling fluid applications this particle size distribution must be optimized with respect to drilling fluid viscosity profile, gel formation and gel fragility, viscoelastic properties as well as chemical properties of the fluid and particles.  In the laboratory there is therefore a need to develop proper equipment to evaluate different selections of added particles.  An example of such a device has been presented by Hettema et al.¹⁵, and this device has been applied to further improve the particle size distribution.