Abstract The strengthening of near wellbore unconsolidated sand formation can play a vital role in reducing sand erosion, sand failure, formation compaction and other borehole problems. The strengthened formation is expected to tolerate higher surge and swabbing pressures leading to a reduction in trip-time failure of unconsolidated and poorly consolidated sand formations. The enhanced formation strength may allow sufficient tolerance to overbalance pressure leading to the use of higher mud weight due to the widening of the mud weight window. Adequate strengthening of the near wellbore formation may also create geo-mechanical condition for underbalance drilling. Different chemicals and polymers can be used either alone or as a part of drilling mud to invade and enhance the near wellbore formation strength characteristics. This paper describes the experimental results of a novel gel system used in improving the strength characteristics of a 16/30 sand pack representing a highly unconsolidated sand formation.
Visual observation shows that the novel gel system is capable to bind the loose sand particle together to create an integrated sand mass with good physio-mechanical consistency. Experimental results show more than 50% increase in the penetration resistance of the sandpack compared to a simulated pore fluid saturated sandpack. The gel has thermal stability up to 100 C. Hence, its inter-particle bonding and strengthening capability will cease for formation having temperature above 100 C. However, the gel could be potential candidate for some improvement of the near wellbore formation strength properties up to a depth having a temperature less than or equal to 100 C. The gel is environment friendly, economically attractive and needs no cross-linking agent for bonding and networking to improve the formation strength.
Introduction Borehole instability arising due to the presence of weak, reactive, fractured, poorly consolidated, mobile, etc. formations cost billions of dollars each year. The non-productive time associated with borehole instability and the cost associated in controlling the borehole instability to an acceptable level are the major factors that increase the drilling cost. Analyses of the nature of borehole problems and the associated loss in productive drilling time indicate that the non-productive time associated with borehole instability problems could exceed more than 50% of the total non-productive time spent in drilling a wellbore. Hence, mitigation of the frequency and severity of borehole instability problems by any means is expected to minimise the non-productive drilling time with a drastic decrease in drilling cost, especially in deep water drilling operation. Even 20% reductions of borehole instability problems will save millions of dollars per year leading to a significant drop in drilling cost.
Hole instability is triggered by the presence of weak, reactive, fractured, friable and unconsolidated formations due to the action of in-situ stresses, physio-chemical effect of poor drilling muds or a combination of them. The presence of weak, unconsolidated and friable sand formations in some oil and gas fields creates borehole problems due to sand erosion, borehole washout, sand collapse, induced fracturing, etc. Unconsolidated and friable sand formations have no or poor inter-particle cementation and cohesive forces to hold the sand particles together. For this reason, the forces causing failure e.g. hydro-mechanical disturbance and in-situ stress change easily exceed the resistance offered by the sand matrix. Strengthening of the matrix of such weak formations by chemicals, polymers or a combination of them is expected to reduce the scope of failure of the formation due to the formation of adequate cementation, inter-particle cohesion or a combination of both. This allows the drilling of weakly consolidated, low strength and troublesome formations as a single hole section leading to the elimination of one or more casing strings and mud change over to reach the reservoir section. The elimination of the installation of even a single casing string could save millions of dollars for a single well. Murray et al. (2002) described the savings of $3 millions by eliminating a 1000 ft casing run and the change over of drilling mud in the Alba field due to a reduction in operational time, cementing, drilling mud cost and cost associated with mud change over, etc. This highlights the high economic benefit of formation strengthening while drilling.