Prevention of Core Jamming and Damage During Coring Operations byOptimization of Drilling Parameters – Analysis of Case Studies

摘要

Coring is a considerably costly drilling operation,but vital to retrieve valuable sources of ground-truth datain the oil and gas industry.To justify the high cost of this operation,essentially,cores should not undergojamming and mechanical damage.Core jamming not only indicates an unprecedented termination of theoperation,but also causes severe damage to the core column already taken in the barrel.This means thatthe cores cannot be retrieved with sufficient key performance indicators(KPIs);thus,it would not reliablyrepresent reservoir properties.Reaching this significant target can be ensured not only by proper design,but also by online monitoring and investigation of the drilling parameters.This investigation predicatedon several case studies in the Middle East to identify the effect of imperfect decisions made by the driller.Based on the experience and observations,the paper provides practical recommendations to alleviate theimpact of such negative actions.The drilling parameters observed in the mud logging data of six coring case studies(of several runsfrom three wells in carbonate reservoirs)were investigated at the depth interval just prior to jammingoccurrence.These parameters consist of rate of penetration(ROP),torque,weight on the bit(WOB),rotaryspeed(revolutions per minute,RPM),circulation rate,and standpipe pressure(SPP).In this work,a maincause of the core jamming occurrences is discussed;that is,the improper drilling parameters application,asmismanaged by the driller.Finally,as indications of jamming,sudden variations of the drilling parametersat the specified depth were investigated.Such study can contributeto the better monitoring for future coringoperations.Based on the observations in six case studies in three wells,WOB was identified as the most significantreason causing core jamming;that is,jamming occurred when the WOB was not steadily maintained onthe bit probably due to the driller's inexperience,shift change,etc.For monitoring,torque decrease,ROPdecrease,and SPP variation or decrease(following initial increases)were identified as indications of corejamming.Torque showed to be a significant parameter forjamming detection when it decreases or its trendbecomes completely straight,i.e.,without any variations,indicating that the core head may not be in contactwith the formation.Next,drilling issues during coring(such as pump failure)can contribute to subsequentjamming.The case studies in this paper provide several observations and indications of core jamming occurrences ina limestone reservoir which can be attributed to non-optimized drilling parameters.The paper also providesseveral practical recommendations and measures to prevent core jamming and core damage.