Optimized Solids Control Provides Opportunities for Drilling Depleted Reservoirs
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 110544, "Optimisation of Solids Control Opens Up Opportunities for Drilling of Depleted Reservoirs," by T.H. Omland, SPE, and B. Dahl, Statoil ASA; A. Saasen, SPE, Statoil ASA and U. of Stavanger; K. Taugbol, SPE, Statoil ASA; and P.A. Amundsen, SPE, U. of Stavanger, originally prepared for the 2007 SPE Asia Pacific Oil and Gas Conference and Exhibition, Jakarta, 30 October-1 November. The paper has not been peer reviewed.
Experience has shown that adding suitable particles to drilling fluid can improve formation strength. This is beneficial when drilling wells with a narrow operational window, as is the case when drilling depleted reservoirs. Successful operations have used several types of particles, including graphite and calcium carbonate (CaCO3). For this approach to work, it is important to establish an optimal particle composition in the drilling fluid. The full-length paper details an offshore operation where coarse shaker screens were used to allow relatively large particles to re-enter the well during circulation. The monitoring equipment allowed close control of the particle-size distribution (PSD) of the drilling fluid flowing into and out of the well during drilling.

Introduction
Drilling through depleted reservoirs is a challenge because the pressure difference between the fracture pressure and the pore pressure is small or sometimes even negative. During the last decade, a technique was suggested that increases the fracture strength. This method is based on fracturing the borehole wall with small fractures and then filling these fractures with impermeable particles to stop further fracture propagation.
Theoretical analysis and analysis of field experience with formation strengthening conclude that it is necessary to optimize the PSD of the added solids. The fracture must be sealed by a nonpermeable, easily plugging material. The plugging is caused by arching or gel formation or a combination of these two. Arching in pipes and conical sections has been a subject for research the last century, and the results are included in most textbooks on soil or powder mechanics. Although this subject is well established theoretically, there still is a need for experimentally optimizing PSD and particle content. Furthermore, this PSD must be optimized with respect to drilling-fluid viscosity profile, gel formation and fragility, and viscoelastic properties as well as chemical properties of the fluid and particles.
Knowledge of what type of particle to be used is necessary to apply this technology successfully. However, it is equally important to control the particle content on an offshore application. Shaker performance and the mechanisms for wear on shaker screens must be known to ensure that only desired particles are returned to the well. The PSD must be quantified. A practical measurement tool has been developed for measuring the PSD of drilling fluids. In the full-length paper, a field test is described where drilled solids and added particles are allowed to be recirculated and thereby naturally achieve formation strengthening. The test is based on the solids-control methods and equipment described in previous papers.