Unconventional reservoirs associated with the presence of fractures have been identified in the Jurassic horizons in the Raudhatain field in North Kuwait. These reservoirs contain high-quality light oil and gas under HPHT conditions with associated H2S and CO2. The operator in this field is presently developing these gas/light oil reserves under the ‘North Kuwait Gas Development Project, The well discussed in this paper is one of a number of new production wells. Historically, Jurassic wells in this field have been drilled using conventional barite-weighted, oil-based mud (OBM) with densities up to 18.5 ppg. However, the challenge to maximize the amount and quality of reservoir data collected and to reduce any impact on the formation caused by OBMs led to a search for an optimized water-based mud (WBM) system as an alternative. A WBM formulated with saturated potassium-formate brine, weighted with manganese tetra-oxide, was selected for the following characteristics: higher base-fluid density, non-damaging nature, robust behavior in HPHT environments, and the possibility of better quality image logs. The well reached 16,530 ft., in a 6–in. hole, with bottomhole temperatures around 280°F. The fluid was thermally resistant, exhibiting very low HPHT filtrate loss and highly stable rheological properties. The maximum fluid density was 16.2 ppg, with no evidence of settling of weight material, even after three days of static conditions. A complete suite of logs was run successfully. The elemental capture spectroscopy (ECS), along with sonic and neutron porosity logs provided good data quality, and the image logs showed excellent resolution. This paper will present all aspects of the planning, design, and utilization of this new drilling fluid. It will focus on lessons learned and conclude with recommendations for further optimization of the design of these drilling fluids.
In one of the prolific fields in Kuwait, achieving zonal isolation posed a big challenge mainly due to setting the production liner shoe close to the oil-water-contact zone. Cement bond logs from the primary cementing jobs were not acceptable due to contamination from intruding water leading to a high water-cut in the produced oil. We review the first implementation of a self-sealing Cementing System in Kuwait to improve zonal isolation and cutting the water production. A comprehensive pre-job study was executed to engineer a suitable cementing system containing a swellable elastomer for oil-water-cuts with proper test in Lab. A novel HPHT multi-function test cell apparatus and procedure were utilized to measure in-situ ability of fractured cement specimens to seal oil-water-flows under the given simulated downhole conditions. Shrinkage or expansion of the set cement was also verified under pressure and temperature with a continuous test method run over several days. Thorough lab tests and Computational Fluid Dynamics simulations were run to enable a fit-for-purpose and robust cement slurry design ensuring proper placement of the cementing system in the well. This paper will describe how this cement was designed and engineered in laboratory. It will also describe how the set up was made simulating a crack in cement specimen and injecting water cut oil reacts and provides desired results. A calculated cement engineering approach was adopted to ensure better cement slurry placement and reduce the chances of slurry contamination. The test conditions were staged to replicate the most appropriate downhole conditions of pressure, temperature and simulated micro channel in the cement sheath. After the successful implementation of the self-sealing cementing system along the 7-in production liner in 2 wells, the corresponding cement bond log images showed hydraulic isolation and the production data from the wells indicated a reduction of nearly 50% in the water cut thus allowing a favorable oil production. This technology is applied in other wells of this field and other fields also with good results. This is being continued to use in critical wells.
Slim Hole Drilling in Kuwait Oil Company (KOC) is not a procedure that is planned for in initial well designs. The complexity of drilling conditions in Kuwait has forced KOC to plan and drill such hole sizes as a contingency. First, one must appreciate the drilling conditions that forces the need for slim hole drilling in Kuwait. Highly fractured, extremely weak zones near surface and below or between several productive field horizons are followed by a pressure transition that ends with a salt/anhydrite section with porous layers interbedded, containing H2S gas and water. In order to control the fluid within such layer, mud weight, in excess of 19.0 ppg is required. Below this section is a slight pressure reversal which includes fractured carbonates. Due to such pressure differences between layers and the position of lost circulation layers within productive zones, numerous casing strings are required to isolate pay zones from weak zones, normally pressured zones from high pressured zones and weak zones from other normally pressured zones, to maintain circulation for drilling and well control. Kuwait Oil Company Deep well programs end with 5 1/2" casing set on bottom which requires that the well starts with 42" conductor at surface and 30" as surface casing. Unanticipated hole conditions or well problems that require setting a casing string at an earlier depth than what was actually planned for in the well casing design, will result in slim hole drilling on bottom in order to reach the objective of the well. Often very little allowance is available for problems by the very nature of drilling conditions. Slim hole drilling in Kuwait is drilled in a hostile environment. The slim hole sections are typically in deep, hot hole sections. This environment often finds high pore pressures virtually near frac strengths. The wells are often either flowing or losing mud. To make matters worse, H2S accompanies any influx or drill volume gas. The paper will cite case histories of wells that have included 4 1/2' slim hole sections at depths from 19300 to 20774' in EX~N-l and over long intervals of 15704 to 18683' in DW-O1 and in mud weight ranges of 16.0 to 19.3 ppg. Slim hole sections have been drilled using downhole motors as well as with rotary drilling. The case histories will additionally describe the down hole tool selections used along with kick and lost circulation handling. Also, discussions will touch on side tracking and completion on DW-O2 where the the original 4 1/2" hole was drilled from 12,620' to 13,800' with 16.0 ppg mud and was side tracked from 12,821' to 13,560'. At the end of the side tracking, 3 1/2" liner was run and cemented as the completion liner and 2.6" bit was run on 1 1/2" coil tubing to clean out inside the liner. Regional Geology Of Kuwait (Stratigraphy & Problematic Formations): During most of geological time, Kuwait appears to have occupied an intermediate shelf area position between the Arabian shield craton to the Southwest. Marine conditions predominate through out the known succession except huge deltaic deposition of sands in the Cretaceous period of evaporate deposition occur in Jurassic and Triassic while a thin tongue evaporates occur in the Miocene of northern Kuwait. There were, however important breaks in the history of deposition when the basin was up lifted and some of the rocks were removed by erosion. It was during the Upper Cretaceous and Tertiary periods when the basin apparently tilted toward the north depositing a thick sequence of these sediments in northern Kuwait. The formations consist of layers of cemented sands, shales, days, limestone (tight & fractured), Anhydrites, salt, etc…. This paper will discuss the problematic formations that are faced during the drilling operation in order to reach the well target. The problematic formations are described in Table - 1 (ranging from top to bottom) Deep Wells Casing Design: Kuwait has a special type of casing design for deep wells.P. 501^
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