The field study is in northern part of Kuwait targeting heavy oil formation, known to be shallow unconventional oil reservoir. It is heterogeneous shallow sandstone reservoir (500ft TVD) with low maturity oil, has low natural pressure, and poorly consolidated. Mud losses known to be the main risk of horizontal drilling in shallow heavy oil environment and the heterogeneous including continuity of the sand are also challenging for geo-steering team in order to place the well in the optimum position. Seismic is not available, however due to high offset well density a good correlation map has been produced. We are using formation tops from offset wells to delineate the continuity of the sand and trend of the structure dipping, we called it as shooting point method, which is assuming the trend of the structure from one offset well to another nearby offset well. The resistivity contrast will be expected to give us around 9 ft depth of detection (DOD) for our Azitrak resistivity tool based on Picasso plot. We made some scenarios for exiting the reservoir and it showed us some early warning 80ft to 180 ft prior to exit the reservoir. We use Autotrak, Azitrak dan Litotrak formation evaluation and density imaging tool to geo-steer and optimally place the wellbore inside 1B sandstone. The expectation of drilling the lateral was below 1000ft MD due to wellbore stability issue. From the correlation of available offset well it is clearly seen, there are two sand bodies in heavy oil target sand. The thickness is around 30-40 ft TVD and the structure was expected to be flat or a little bit dipping down. The well was landed in the middle of 1B, based on correlation of actual landing point log data to the nearest offset wells. Distance to bed boundary (D2B) showed local conductive layer from bottom since drilling the lateral section, which was not the response of base of 1B sand. So it was recommended to go down in stratigraphy in order to place the trajectory at the bottom part of 1B sand. In order to minimize wellbore stability issue along the lateral section, Bakerhughes recommended to maintain consistent faster ROP (80-100ft/hr) and effective hole cleaning. In the middle of lateral section of well B (1750ft MD) the well trajectory was inverted for the optimum production purposes to total depth (2250ft MD). Total lateral length achieved is 1116ft MD which covers 100% of the lateral length. Shooting point method in defining the rough structure trend from one well to another well was effectively applicative in the field, where current structure after drilling the lateral section is almost flat or slightly dipping down same as predicted before.
Lower Fars is a shallow unconsolidated sandstone reservoir with high inter-granular porosity filled with heavy oil in southern part of Ratqa Field in Kuwait. The shallow depth (500' to 800'), friable sand laminated with shale and high viscosity heavy oil in pores have made coring this sand quite challenging. Last recovered core with rubber-sleeved core barrel in the eighties was only up to 60%. This paper describes how thoughtful mix-up of technology, innovative techniques and proper coordination by aligning all concerned has helped in meeting the challenge of coring unconsolidated sand and it's processing. Using low invasive core fluid, shorter core length, Aluminum inner core barrel, separate core bit to cut major sand / shale, full core catcher system, vertical slabbing at well mouth and on-site freezing have improved core recovery in excess of 85%. While low-solid content coring fluid with a pH of 9 resulted in low mud invasion, reduced WOB, ROP and SPM ensured fewer washouts during coring. Low abrasive core head with clam shell full closure core catcher produced good recovery. Core barrel length was reduced from standard 30' to 10', which was slabbed to 3' size keeping barrel vertically at well mouth. At well site it was frozen vertically with foam on top to minimise lateral movement and transported in freezer. At Core Lab it was kept frozen with dry ice, slabbed and plugged with liquid N2. It is expected that the obtained core plugs from similarly cored 6 wells shall lead to meaningful Routine and Special Core Analysis, which was suspected in old cores. This would help in developing the depositional geological model in conjunction with the image logs. Introduction The success of core analysis depends largely on how good the coring operation and well site core preservation are made. Conventional Coring in the unconsolidated formation is always problematic. The shallow sandstone reservoir filled with heavy oil in southern Ratqa Field of Kuwait is a challenge by itself. Earlier attempts in the eighties with rubber-sleeved core barrel had resulted in maximum recovery only up to 60%. When new wells were drilled as part of development plan, it was decided to core some early wells using modern coring technology. Low invasive core fluid, shorter core barrel length, Aluminum inner core barrel, separate core bit to cut major sand / shale and full core catcher system had helped in core recovery in excess of 85%. While low-solid content coring fluid resulted in low mud invasion, reduced WOB, ROP and SPM ensured fewer washouts during coring. Low abrasive core head with clam shell full closure core catcher resulted in better recovery. Core barrel length was reduced from standard 30' to 10', which was slabbed to 3' size keeping barrel vertically at well mouth. At well site it was frozen vertically with foam on top to minimise lateral movement before it was transferred to a chest freezer. The freezer was transported in running condition to Core Lab where it was kept frozen with dry ice for atleast 2 weeks. Core slabbing and plugging were performed using liquid N2. The obtained core plugs helped in meaningful Routine and Special Core Analysis, which was suspected in old cores. The good core recovery had also helped in characterization of depositional facies leading to the geological model in conjunction with the image logs. Unconsolidated sand coring- a new ball game The main areas of concern for unconsolidated formation coring are:minimising mud invasionusing proper coring assembly so as not to lose low strength rock capture during coringcore handling process so as not to jeopardize core preservation andnon-reactive preservation material.
Kuwait has an unconsolidated formation with viscous oil at shallow-depth. Drilling and completion of horizontal well at such shallow depth is quite challenging. Industry practice is to use Slant Rig for shallow wells. Drilling experts preferred solution that would not entail Slant Rig for any future interventions and instead suggested using conventional vertical rig to establish feasibility of drilling and completion as a pilot. Lots of pre-drilling studies were carried out which involved Geomechanics Study to understand Stress orientation, pore pressure and sanding risk; Laboratory test of return permeability on core plugs for drilling fluid design and pore-bridging material selection; XRD & SEM analysis for clay mineral identification; Torque and Drag Analysis for predicted performance during drilling and completion and Particle Size Analysis for sand control design with slotted liner. Pre-job coordination meetings, Daily briefings, usage of Rotary Steerable System, Geo-Steering and Mid-course correction of trajectory based on Real Time Data Monitoring resulted in well placement in sweet spot. Well by well, rig days got reduced, lateral lengths increased, tangent sections for pump placement optimised and practically no held up occurred during drilling, casing/liner lowering or completion. This was a World record of drilling horizontal well using vertical rig at such shallow depth. The well completion as verified from Silicon Activation log suggests optimal placement of the slotted liner. This is further vindicated from zero sand and water production, another great achievement for the pilot project. Till date 7 such shallow wells have been drilled with 6 on continuous production for over 2 years in different parts of the field. Successful drilling and completion of these shallow-depth horizontal wells by conventional vertical rig in an efficient and cost-effective manner has confirmed all the pre-drilling assumptions and technological tests for the pilot phase, reaching world record breaking achievement.
The Miocene age shallow unconsolidated sandstone formation in Kuwait contains viscous oil whose distribution is primarily controlled by depositional facies and their diagenetic modifications. This study integrates core-calibrated resistivity images with openhole logs to obtain high-resolution facies logs with continuous array of oriented depositional structures, facies types, and environment-related facies associations with paleocurrents measurements. These are combined in defining reservoir zonation, geometry, spatial distribution and paleo-geographic evolution during its accretion across the field.The formation is characterized by variable grain sizes, sedimentary structures and diagenetic cementations. The sandstone succession is punctuated by scouring surfaces and separated by discrete mudstone intervals mixed with scattered bioclasts. The upper section of the formation encompasses two main oil-producing sandstone intervals (S1 & S2 reservoirs), separated by a mudstone unit (Mid Shale) and sealed on top by marine Cap Shale unit. Each sandstone interval consists of lower sheet-forming stacked horizontal-and cross-stratified sandstone units (S1B & S2B) and upper lenticular sand-prone bodies (S1A & S2A) that laterally truncate or intermingle with units of sandy mudstone and argillaceous sandstone (S1 Shale & S2 Shale).The observed physical and biological features, paleocurrents data and vertical hierarchy of recognized sandstone and mudstone facies suggest the oil-bearing sequence formed as fluvial to upper delta-plain deposits that were accreted by repeated lateral shifting, and an overlapping of fluvial channels, distributary/abandoned channels, crevasse splays and inter-distributaries lakes or bays of estuarine origin. The active fluvial-and distributary channels were generally flowing from SW to NE, and locally due E and SE, and largely discharged from S. The morphological parameters reveal that the fluvial channels migrated laterally and bifurcated downstream, and fluctuated across a low sinuous channel belt. With continuous base level rise, the area was drained entirely by transgressive brackish-water lacustrine or bay events depositing the widely distributed Mid-Shale and Cap-Shale units.
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