Estimation of Formation Strength From the Mechanical-Properties Log(incudes associated paper 6400 )

Abstract

The mechanical-properties log provides a quantitative means for identifyingsands that are strong enough to produce oil and gas without any form ofsand control. The method is based on a correlation of in-situ strengthwith the dynamic elastic moduli computed from sonic and density logs. Introduction To meet current demands for more oil and gas, companies would like to increase production. However, increases in well-production rates are frequentlyaccompanied by sanding problems. For this reason, there is often reluctance to go to higher producingrates.Yet many sands are, in fact, strong enough topermit greater production. It would be highly beneficialto be able to recognize these. It is also important toknow beforehand which sands are too weak to beproduced at the higher rates because once the wellhas made sand, consolidation is difficult and oftenineffective.Many factors must be considered to understandthe sanding problem. The pressure gradient near theperforation, the flow rate per foot, and the scrubbingaction of the fluid being produced all interact toimpose destructive forces on the sand. The ability ofthe sand to withstand these destructive forces isdetermined by two main factors: the intrinsic strengthof the formation, and the capability of the sand toform stable arches around the perforations.The intrinsic formation strength is governed bythe state of the confining stress (as determined bythe difference between the overburden stress and thepore pressure), the grain shape and sorting, and thecementation between the grains. Shaliness maycontribute to the cementation.There is a good correlation between theformations intrinsic strength and the ability of theformation to produce at a high flow rate. Moreover, the consistently lower permeabilities of the strong sandslimit the magnitudes of the flow rates to which theyare exposed. Conversely, weak sands have relativelyhigh permeabilities and are capable of producing athigh flow rates, even with small drawdowns. Forthese, sanding is a potential danger.Although we believe that a criterion based on theintrinsic strength is basic and fundamental, there areother factors that cannot be ignored. For instance, the type of fluid being produced is important. Ourexperience to date has been predominantly with gasand oil production, but it is believed that productionwith a high water cut may require higher intrinsicstrength.Resistance to sanding in a weakly cemented sandcan result from the formation of a stable, load-carryingsand arch spanning the producing cavity. In this respect, published work on laboratory experimentsindicates that the presence of two different fluids(e.g., oil and water) in the formation, with the wetting-phase saturation near irreducible, maycontribute cohesive forces between the sand grains.These forces help sustain a stable arch after some sanding has occurred. The forces result from theinterfacial tension between the two fluids where theirsurface contacts the sand grains, and the effect is topull the sand grains together. JPT P. 283^