Besides satisfying the basic requirements of scientific drilling construction, drilling fluid should satisfy the requirements of scientific test, well logging and borehole log, etc. Detailed requirements can be found as follows:
- No chemical composition which may affect scientific test, well logging and borehole log;
- Small influence to environment safety;
- Good lubrication and low cost;
- Effectively relieving complex downhole situation, including high temperature and high pressure;
- With strong weighted bearing capacity, including inverting into salt drilling fluid system;
- Low solid content, with less interference to well logging, acoustic transmission and visual reception, avoiding jam of drilling tool;
- With strong capability of carrying debris (cuttings), and can produce thin and good tenacity mud cake.
Strata Encountered and Requirements of Well Structure
Main strata in CCSD-1 Well are eclogite, paragneiss and orthogneiss. Lithology of these strata are not of water sensitivity, and their permeability are extremely tiny, so there was no special requirement for inhibition capability of drilling fluid to water sensitive layers except some broken borehole sections with collapses and block dropping. Hardness and abrasiveness of those strata are high, the cuttings produced in drilling were extremely tiny and with large density from 3.5 to 3.8 g/cm3, so good lubrication of drilling fluid was required to alleviate the wear of borehole wall to drilling tool. Because of the high density and tiny size of the cuttings, good carrying and suspension capabilities were required for drilling fluid to carry the cuttings produced in drilling out of hole in time, and then to be removed by solids control equipments, so as to improve drilling efficiency and avoid bit bouncing and bit burying. Cuttings produced in diamond drilling can not be removed by normal solid control equipment for oil drilling, so high efficiency solid control equipment and related methods must be employed for solid removal.
Though most borehole sections encountered in drilling were integral and there was no formation pressure in those integrated crystalline rock strata, several broken zones and leakage zones were still encountered in drilling. Leakage problem was solved by sealing with inert material, and circulation of drilling fluid could be maintained, however, repeated leakage might be easily happened in drilling because leakoff pressure in those sealed borehole section was still small. So density and rheological parameters of drilling fluid should be strictly controlled to decrease the circulation pressure of drilling fluid.
In drilling construction, performance of drilling fluid and parameters of drilling technology are related to borehole tructure. Flow state of drilling fluid should be kept constantly and abrupt change of local flow state should be avoided in drilling. Usually, “one sized casing extends to hole bottom” is impossible, several steps of casing need to be run in hole, so drilling fluid can easily form turbulence at the area between open borehole and casing, leading to an accumulation of cuttings. Well accident such as bit burying and bouncing caused by abrupt slumping of the cuttings accumulated to a certain amount will happen. Influence of drilling fluid flow state to drilling status is very obvious, too, not only related to suspension and carry of the cuttings, but also to the stability of borehole wall, circulating pressure loss of drilling fluid and operation conditions of downhole power drilling tools.
During the period of core drilling in the pilot hole, in order to ensure the carry of the cuttings, 244.5 mm (958 in.) moving casing was run in the 339.7 mm (1338in.) casing, with the main purposes of improving the flow state of drilling fluid and improving uplift velocity. Even so, because borehole diameter of core drilling was 157 mm, the inner diameter of 244.5 mm moving casing was 222.4, and 89 mm (312 in.) drill stem was employed for core drilling, so the uplift velocities of drilling fluid in two borehole diameters differed 2.5 times. During the period of core drilling in the main hole, though 193.7 mm (758 in.) moving casing was set in the borehole above 2019 m, the difference of uplift velocities was still comparatively obvious. So, carry of cuttings in different hole diameters must be considered when designing the properties of drilling fluid.
Requirements of Core Drilling
PDM and hydro-hammer downhole power drive core drilling method was the main way employed for CCSD-1 Well. Drilling fluid not only just worked as normal mud, but also provided working medium and power to the downhole engine (PDM and hydro-hammer). The working parameters of those downhole power drilling tool assemblies were determined by discharge rate of drilling fluid, at the same time, quality of drilling fluid properties would directly affect the working quality and service life of the downhole power system.
There is a close relationship between operating characteristics of PDM and circulation pressure and discharge rate of drilling fluid. The output torque is proportional to pressure difference between motor inlet and outlet, and output rotary speed is related to the discharge rate through the motor but no relation with pressure difference basically. So, when designing drilling fluid parameters, especially for drilling parameters or discharge rate, it was very important and should be the key element to determine the discharge rate of drilling fluid according to rotation speed of diamond drilling at first. Uplift velocity and circulating pressure drop are determined by discharge rate, and carrying capability is determined by uplift velocity and rheological parameter of drilling fluid. After PDM core drilling roundtrip is finished, circulation for hole flushing is forbidden before lifting the drilling tool in case of core loss, and 10 min gel strength value of drilling fluid should be increased to improve its suspension capability just in case of cuttings sedimentation.
Percussive power and percussive frequency of hydrodownhole-hammer are determined by the pressure difference between inlet and outlet of the hammer and the discharge rate of drilling fluid. Conventional valve type hydro-hammer is very susceptive of quality and performance of drilling fluid, so, when viscosity of drilling fluid is high, there will be a comparatively big loss of pressure between the two ends of the hydro-hammer, and this will affect single stroke percussive power of the hydro-hammer. Solid content, especially sand content of drilling fluid is very unfavorable to hydro-hammer, especially to valve type and fluidic type hydro-hammer. Drilling fluid with high sand content can seriously erode valve body and fluidic element in high speed of flow behavior, causing a premature wear or even abandonment of the hammer.
Allowable working discharge rate of hydro-hammer in drilling is limited. In hydro-hammer drilling, the range of this discharge rate should be fully considered to ensure effective work of the hammer and the carrying capacity of the cuttings. The range of working discharge rate of PDM should be considered, too when PDM hydro-hammer drilling is employed. In addition that PDM and hydro- hammer both have high speed moving parts, so lubrication property of drilling fluid is very important to improve efficiency and service life of drilling tool. Thus, PDM hydro-hammer core drilling has more requirements for drilling fluid in comparison with conventional rotary drilling.
Requirements of Non-core Drilling and Expanding Drilling
The size of the cuttings produced in non-core drilling and expanding drilling is comparatively large with diameter of 2–5 mm as a result of roller bit is employed. Especially in some broken borehole sections, even larger cuttings are usually produced because of dropping blocks from borehole wall, so yield point value of drilling fluid should be improved in drilling fluid treatment.
Rotary table drive is employed for non-core drilling and expanding drilling, all the drilling tools will be rotated in borehole. When hole deviation is large, the wear of hole wall to drilling tool will be rather serious because no mud cake is produced on hole wall as a result of extremely small permeability of strata. Thus, lubrication property of drilling fluid should be improved in drilling fluid treatment.
The time used for non-core drilling and expanding drilling in each roundtrip is comparatively long. The temperature of drilling mud at surface will be higher than 40 °C and the temperature in hole is even higher because of the long time continuous circulation of drilling fluid in deep well, especially in summer. At that time, the polymer in drilling fluid will be degraded, not only producing a large quantity of foam, but also changing the performance of drilling fluid. Thus, problem of anti-corrosion should be solved in the process of drilling fluid treatment.
Requirements of Borehole Log
Accurately and completely acquiring the core, liquid and aeriform samples released in drilling with modem scientific and technological means is one of the main purposes of CCSD-1 Well project. And providing the basic data for geological research, long term observation and other subject study is the task of borehole log. Geological logging data and follow-up monitoring of compound logging instrument can also provide technical support for drilling project and ensure that the project can be successfully implemented.
Borehole log methods used in CCSD-1 Well included core logging, cuttings logging, drilling fluid logging and follow-up monitoring with compound logging instrument, all those methods were related intimately with the performance and components of drilling fluid. Variations of every kind of ion and gas in drilling fluid could be detected by different liquid and gas analytical and detecting instruments in time. Testing methods and technical descriptions of drilling fluid samples are shown in Table 1.
GC-MS, GC, IC and LC were employed for accurate surveying of microvariations to each component in drilling fluid.
After full hydration, components of drilling fluid in drilling process are basically steady and will be the background data of borehole log. Drilling fluid should be adjusted many times as a requirement of drilling project. So, drilling fluid components after adjusting need to be tested before running in hole. While drilling fluid is run in hole, element metathesis and chemical combination or even new crystalline minerals may occur after actuating with the rocks in open hole section in circulation. When abnormal phenomena happen in borehole log, it means that something of
stratums has intruded into drilling fluid.
Thus, following requirements must be satisfied when drilling fluid is prepared and disposed to ensure the veracity of borehole log and explanation of abnormal phenomena.
- Components of drilling fluid are known;
- Components of drilling fluid should be as simple as possible;
- Treatment times of drilling fluid should be as least as possible;
- Drilling fluid system should have high temperature stability and higher resistance to fouling;
- No components which may influence borehole log;
- Intimate coordination with loggers in treatment progress of drilling fluid.
Requirements of Environmental Protection
Along with the improvement of human living standard and advance of science and technology, the awareness of mankind to environmental protection is increasing, too. Usually, drilling construction will cause a certain influence to ambient, such as noise pollution, groundwater fountain pollution, discharge pollution of construction waste liquid and domestic pollution and so on, in which, discharge pollution of construction waste liquid is more serious. Drilling fluid treatment of CCSD-1 Well should satisfy the following requirements for environmental protection:
- During construction, discharge of waste water and trash must be strictly controlled, waste drilling fluid from field construction, debris from hole and trash from well site need to be innocuously treated.
- When the project is finished, all the waste drilling liquid must be left in mud pit and all with debris will be exported and buried after evaporation and concentration under natural conditions.
Requirements of Drilling Fluid Design
Based upon the above-mentioned situations, following requirements to the drilling fluid system of CCSD-1 Well was proposed:
- Should not contain chemical materials which might influence well logging, borehole log and scientific experiment;
- With the performances of low viscosity, low shear strength and shear-thinning property to decrease the circulating pressure drop;
- Stability of temperature resistance should be larger than or equal to 150 °C;
- With good lubrication property to reduce drilling torque, and to improve the service life and reliability of the down hole engine;
- With good carrying capacity, especially for the upper borehole section where the uplift velocity in the annulus is rather slow;
- Should be a good carrier to transport energy for down hole power system besides acting as normal drilling fluid;
- With good fluidity and low flow resistance to meet the working requirement of PDM and hydro- hammer;
- With sand content as low as possible to decrease erosion and wear of drilling fluid to PDM and hydro-hammer;
- With good suspension capacity to avoid cuttings depositing;
- Non-corrosive to metals;
- Be favor of solid control;
- With strong capability of invasion resistance.
Based upon above-mentioned requirements, drilling fluid system with low solid and low molecular polymer was to be adopted for CCSD-1 Well drilling, in accordance with the method of core drilling to be used, strata to be encountered and the hole structure designed, with the main technical descriptions shown in Table 2.
Artificial sodium bentonite or LBM-SD was to be added in the drilling fluid for core drilling to regulate its performance, so as for non-core drilling and expanding drilling (Table 3).