Thirty percent of the deepwater wells in the western South China Sea have high-pressure and high-temperature (HP/HT) characteristics. After incorporating analyses of engineering and geological environments for deepwater HP/HT wells in the Lingshui Block, the authors suggest in the complete paper that the solution for drilling problems in deepwater HP/HT wells should start with drilling fluid. Researchers have developed a novel water-based-drilling-fluid system compatible with deepwater HP/HT wells in the Lingshui Block on the basis of a conventional drilling fluid and further optimization.
Introduction
Most of the reservoirs in the South China Sea are not only deeply buried with a low seabed temperature but also have HP/HT characteristics in the lower part of the well interval. The low temperature of the seawater environment causes an increase of drilling-fluid viscosity in the riser and deteriorates the rheology, leading to a buildup of equivalent circulating density (ECD) that might induce lost circulation. HP/HT reservoir conditions will also result in a narrow drilling-fluid window. The drilling fluid has to be HT-resistant and must prevent overflow and wellbore instability, as well as increase the safe drilling-fluid window.
The Lingshui deepwater gas field in the western South China Sea is a typical HP/HT reservoir. Its greatest operational water depth is 1688 m, and its highest well temperature is 167°C. The estimated reservoir temperature of the Lingshui Block is 176°C, with the maximum pressure coefficient reaching 2.01.
Challenges Encountered in Use of Drilling Fluids in the Western South China Sea
The major challenges encountered in the use of drilling fluids in deepwater HP/HT wells in the Lingshui Block can be placed into two general categories as follows:
1.The deep water and HP cause further narrowing of the drilling window.
2.The coexistence of HP and HT makes the drilling-fluid performance difficult to maintain.
The existence of a water interval of a few hundred meters or even a few kilometers in deepwater blocks usually forms a 2 to 4°C low-temperature interval from the seabed up to 500 m below sea level. The low-temperature environment gives rise to rheology variations of drilling fluid in the riser, which may cause a gelling effect with increasing viscosity and density, producing a higher friction in the wellbore flow and increasing the risk of formation leakage at the casing shoe. Conventional deepwater wells usually only focus on low-temperature resistance without considering the effect of HT. However, HT will cause degradation and crosslinking of certain drilling-fluid additives, weakening the antipollution characteristics and the water solubility of the additive.
During the drilling operations in the HP/HT deepwater Lingshui Block, especially when drilling in the reservoir formation, the drilling fluid not only had to prevent the occurrence of the drilling-fluid gelling effect under low temperatures but also had to prevent the degradation of drilling fluid in the HT interval, which may greatly reduce cuttings-carrying capability. Therefore, deepwater HP/HT drilling can face more technical challenges than conventional deepwater drilling does.
These technical challenges had never been never encountered before in the western South China Sea. Several commonly used drilling-fluid systems could not fully meet the requirements of the compound/complex geological environment of Lingshui deepwater HP/HT wells. It thus became necessary to develop a novel drilling-fluid system for the deepwater and HP/HT geological -environment of the Lingshui Block.
Construction of Deepwater HP/HT Drilling-Fluid System
To prevent drilling-fluid thickening at low temperatures, low-molecular-weight polymer is widely used to control rheology properties in offshore drilling. Flow rates of mud pumps and booster pumps are increased to keep the wellbore clean. In addition, hydraulic software is used to perform real-time simulation and calculation during the operation in order to monitor fluctuations of ECD. Furthermore, hydrate will also affect the rheological properties of drilling fluid and cementing quality, which will influence operational safety. Currently, thermodynamic inhibitors are primarily used to prevent the generation of hydrate in the process of drilling deepwater wells. Species and quantity of inhibitors can be obtained by simulation through laboratory experiments or software.
Construction Scheme
Stemming from the technical challenges encountered by deepwater HP/HT drilling fluids, and with an existing conventional deepwater drilling-fluid system as a basis, an HT-resistant filtrate reducer was introduced. Meanwhile, the hydrate-inhibition ability of the entire system was considered. In this way, construction of a drilling-fluid system suitable for deepwater HP/HT wells could be achieved.
Low-Temperature-Resistance Scheme
Current measures to inhibit gas hydrates include semi-inhibition: inhibiting the generation of hydrates during the drilling process. Inorganic salt and ethylene glycol are the most important hydrate inhibitors. Furthermore, potassium salts can significantly improve the temperature-resistance capability of polymer. Thus, the researchers selected NaCl and a potassium formate compound (KCOOH) as hydrate inhibitors. According to the software simulation, when 5% NaCl and 10% KCOOH are deployed together, the critical temperature for hydrate generation is 14°C at 15 MPa and 15°C at 20 MPa, reaching the requirement of semi-inhibition. The critical temperature of the system at 20 MPa is 16.63°C; this can effectively inhibit the generation of hydrates.
HT-Resistance Scheme
The optimization of an HT-resistant filtrate reducer for deepwater HP/HT drilling fluid should follow the principles of steady rheology, low filtration loss, extensive sources, and acceptable cost. The imported HT-resistant filtrate reducer DT and the domestic reducer HTFL were evaluated. The results show that the viscosity effect of HTFL in the system is minor, while its filtration loss at HP/HT conditions is similar to that of DT, meeting the requirements of the HP/HT geological condition in the deep-water Lingshui Block. In this case, HTFL was selected as the HT-resistant filtrate reducer.
Drilling-Fluid Formula
On the basis of the early optimization of a single additive, HT-resistant filtrate reducer HTFL and polysulfonate HT-resistant filtrate reducers SMP HT and SPNH were selected to conduct filtration-loss control of the system. NaCl and the potassium formate compound HCOOK were selected as hydrate inhibitors. The inhibitor CP1 consists of three types of polymer compounds. The lost-circulation material CP2 consists of three types of material with different particle sizes. Experiments were initially performed according to formulae with pressure coefficients of 2.1 at 180°C. The formulae and results gathered in comparing the HT-resistant filtrate reducer HTFL with DT are shown in Tables 2 and 3 of the complete paper. Formula 3 was chosen for the deepwater HP/HT drilling-fluid system.
In the experiment, rheology and filtration loss under HP/HT conditions were considered with the use of HT-resistant filtrate reducers HTFL, SMP HT, and SPNH HT. As the dosage of HTFL increases, the viscosity of the system increases and the amount of filtration loss reduces. When the dosage of the temperature-resistant filtrate reducer is 0.8%, dosage of CP1 is 2%, and dosage of CP2 is 5%, the rheology of the system remains stable, HP/HT filtration loss is less than 10 mL, and mudcake quality is sound.
Performance Evaluation of the System
The rheology of the deepwater HP/HT drilling-fluid system was evaluated in low- and medium-temperature and HT conditions. The results show that the rheology of the system from low temperature to HT is stable. As the temperature increases, the viscosity of the system tends to be steady and the trend of the yield point is also stable, meeting the technical requirements of deep-water HP/HT drilling fluids in the Lingshui Block. The system also demonstrates an antipollution capacity.
A dynamic filtration-loss instrument was used to evaluate lost circulation and pressure-holding capability of the drilling fluid at 10 MPa and 150°C, and the filtration loss was recorded at a different time. The filtration-loss rate of the system became stabilized and was less than 0.08 mL/min after 15 minutes. This indicates that the system has formed highly effective sealing in the core cross section, preventing solids and filtration from entering the core. The system also can increase the formation-pressure-holding capability to some extent. Sag stability is also seen to be stable.
According to a standard laboratory-testing method of evaluating formation damage by drilling and completion fluids, results show that the -permeability-recovery factor of the system is greater than 89.26% and the formation protection of the system is excellent.
Researchers acquired 22 pieces of slough from the second member of the Yingerhai Formation, which is rich in shale and which used base mud to conduct soak tests to evaluate the inhibition of shale hydration. Although several cleavages can be seen after soaking 120 hours, the slough still keeps its integrity well, which indicates that this type of drilling fluid is capable of inhibiting shale from being hydrated as a result of soaking and can maintain wellbore stability in the Lingshui Block.
Conclusions
1.A drilling-fluid system compatible with deepwater HP/HT wells features a pressure coefficient of 2.10 at 180°C. Its rheology at low temperature is steady, and hydrate does not appear in the operation. It also has good sag stability, strong antipollution properties, and good formation-pressure-holding and reservoir-protection ability.
2.The optimized drilling-fluid system could also enhance the formation-pressure-holding capability to a certain extent.
3.The operation went smoothly and without complex situations caused by the deepwater HP/HT conditions, with an average operational period of 42 days (with an average 4400-m well depth), average timeliness of 98.97%, and average nonproductive time of 8.6 hours.
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