O Overproduction of water and sand is one of several common issues in producing oil. Analysis towards this problem becomes a valuable asset in maximizing oil recovery. Based on this reason, writer pursues research of sandproblem and water coning at Sanga-Sanga Block, East Kalimantan. Analysis asthe evaluation is targeted on reservoir by certain limitations such as lenses reservoir characteristic with deltaic precipitation environment which was drilledvertically through several reservoirs. In general, the reservoir is sandstone whichhas combination drive mechanism which is dominated by water drive.This research is a further study which is needed by PT. Pertamina HuluSanga-Sanga (PHSS) as an evaluation in hydrocarbon reserve utilization. Studycompleted under the supervision of company’s terms and conditions by providingdata. Evaluation process initiated with well selection which has characteristic similarity and suspected has water coningand sand problem.Then, calculation is held towards critical rate that will be further analyzedwith Chan’s Diagnostic Plot by comparing WOR/WOR’ versus time. From theplotting, later we will able to differentiate which well encountered water coningand/or just water channeling In water coning analyses, observation was done towards production data,spesifically water cut where significant rise marks the existence of water coning.Then, Chan’s Diagnostic plot was conducted by comparing WOR/WOR’ to timewhich will be analyse in advance by the critical rate. From the plot, later on willbe able to differentiate wells that experienced water coning or water channeling.The result will be validated with the critical rate calculation. There are severalmethods on calculating critical rate in vertical well, which are Meyer-GarderMethod, Schols Method, Chaperson Method, Hoyland-Papatzacos-Skjaeveland Method, Chierici-Ciucci Method dan Chaney et al Method. If production rate oractual rate over the critical rate then it divined the existence of water coning inthe reservoir. Last, by the method of Sobocinski-Cornelius will be calculated thebreakthrough time.In sand problem analyses, production observation conducted such ascementation factor, shale volume, and formation strength. Where all of thaanalysis then will be validated by calculating the critical flow rate. At the end of the research will be obtained an evaluation towards existingwells. Whether the well experience water coning or water channeling byproviding critical rate data. Also, it will be able to estimate when will be the water coning happened by the calculation of breakthrough time. On the otherhand, it will identify the exixtance of sand problem..