China's underbalanced drilling technology can be traced back to 1960s. Since 1990s, China's underbalanced drilling technology has been accelerated, especially the successful implementation of Jiefang 128 well and Lungu series wells in Tarim Oilfield, which pushed China's underbalanced drilling to a new stage.
3.3.4. 1 Classification and classification of underbalanced drilling technology
(1) underbalanced drilling type
According to process classification: it can be divided into liquid phase (water-based, oil-based drilling fluid) and gas phase (air, nitrogen, atomization, foam and aeration). The density corresponding to underbalanced drilling technology is:
1) For gas drilling, the suitable density range is 0 ~ 0.02g/cm3.
2) For atomized drilling, the suitable density range is 0.02 ~ 0.04g/cm3.
3) For foam drilling, the suitable density range is 0.04 ~ 0.6g/cm3；; When the wellhead is back-pressured, the applicable range of density can reach above 0.8g/cm3.
4) For aerated drilling, the suitable density range is 0.7 ~ 0.9g/cm3；; Some areas are even higher.
5) For drilling with water-in-oil or oil-in-water drilling fluid, the suitable density range is 0.8 ~ 1g/cm3.
6) When drilling with fresh water or salt water drilling fluid, the suitable density range is 1.0 ~ 1.30 g/cm3.
7) For conventional drilling fluids, the applicable density range is greater than 1. 10g/cm3.
8) Mud cap drilling is used to drill deep high-pressure fractured reservoirs or gas reservoirs with high hydrogen sulfide content.
At present, there are several kinds of underbalanced drilling technologies: gas-phase underbalanced drilling, gas-liquid two-phase underbalanced drilling and liquid-phase underbalanced drilling.
(2) Classification of underbalanced drilling
In order to choose the appropriate equipment and corresponding scheme for engineers and technicians, the Underbalanced Operation Committee of American Drilling Contractors Association has formulated the classification system standard of underbalanced oil wells. The classification system standard divides risky oil wells into 6 grades, from 0 to 5. Each level is divided into two categories, A and B, which are classified as follows:
Grade 0: only improve drilling efficiency, not involving oil and gas reservoirs.
1 grade: the oil well cannot flow to the wellhead by its own pressure, so the oil well is stable and the risk is low from the perspective of well control.
Grade 2: The oil well can flow to the surface under its own pressure. In case of catastrophic equipment failure, conventional well killing methods can be adopted.
Grade 3: Geothermal wells that do not produce oil and gas. The maximum shut-in pressure is less than the bearing capacity of unbalanced equipment, which will lead to serious consequences if catastrophic equipment failure occurs.
Grade 4: crude oil is produced, and the maximum shut-in pressure is less than the working pressure of unbalanced equipment. In the event of catastrophic equipment failure, it will immediately lead to serious consequences.
Grade 5: The maximum injection pressure is greater than the under-balanced operating pressure, but less than the maximum bearing capacity of the BOP. Catastrophic equipment failure will immediately lead to serious consequences.
Underbalanced drilling equipment and technology in 3.3.4.2.
(1) Conventional underbalanced drilling equipment
1) Surface equipment: including rotary BOP, single (double) ram BOP, throttle manifold, four-way, hydraulic control valve, liquid-gas separator, killing heavy mud tank, skimming tank, oil storage tank, various high (low) pressure hard (soft) pipelines, flashback prevention device, ignition pipeline, automatic igniter, etc.
2) Downhole tools: including arrow-shaped check valves, throw-in check valves, drill pipe upper (lower) cocks, hexagonal drill pipes, bypass valves, etc.
3) Other equipment: including wireless communication equipment, harmful gas alarm and protection equipment, fire-proof and explosion-proof equipment, etc.
At present, the equipment that can complete the whole process of underbalanced drilling can be divided into two categories. One is the wellhead forced tripping device, which is used to overcome the jacking force in the well at the end of tripping or the beginning of tripping; The second is the downhole packer or downhole casing valve, which can complete the downhole shut-in and make the wellhead complete the pressure-free tripping of the pipe string.
(2) Gas drilling equipment
1) Equipment capacity requirements. Gas (atomizing) drilling equipment needs different capabilities according to the size and depth of the borehole, the size of drilling tools used and the water production of the borehole, and needs to be analyzed according to the specific situation to determine the type, parameters and capabilities of the equipment. See Table 3-8 for the results of gas drilling parameters calculated by gas drilling calculation software.
2) Composition, function and flow of gas (atomizing) drilling equipment. In addition to the wellhead pressure control equipment-rotary blowout preventer, gas drilling also needs some special equipment. Different combinations of these equipments can meet the requirements of different drilling methods (Table 3-9). See Figure 3- 1 19 for gas drilling equipment and circulation flow.
Underbalanced drilling design in 3.3.4.3.
(1) general principles
The value of bottom hole negative pressure varies from region to region, and is comprehensively considered according to the oil and gas production under the unit pressure difference of adjacent wells, the processing capacity of surface equipment, the estimated oil production while drilling, the requirements for wellbore stability, the operational stability of field equipment, and the professional level of construction personnel. In the design, generally follow the following principles:
Table 3-8 Gas Consumption for Gas Drilling under Different Well Conditions
Table 3-9 Equipment Required for Gas Drilling
Fig. 3- 1 19 gas drilling circulation flow chart
1) The lower limit of bottom hole negative pressure value is zero, and the upper limit is the difference between formation pore pressure and formation collapse pressure.
2) The bottom hole negative pressure value of liquid-phase underbalanced drilling technology should be designed as small as possible to reduce the wellhead pressure. Generally, the bottom hole negative pressure value is between 1 ~ 3 MPa.
3) For gas and atomizing drilling, the bottom hole negative pressure value is not specially designed.
4) For foam and aerated drilling, there is a large space for designing the bottom hole negative pressure value, and there is gas phase, so the bottom hole negative pressure value can be designed to be larger to prevent overbalance. When the riser is aerated drilling, the bottom hole negative pressure value should be greater than 2MPa.
5) Negative pressure difference is an important parameter to ensure the success of underbalanced drilling. The design of negative pressure difference should be comprehensively considered from wellhead equipment, casing bearing capacity, rotary control head performance, borehole stability, ground separation ability of produced fluid and so on.
(2) Gas drilling design
1) Selection of air drilling section. The mechanical stability of formation, gas, water and hydrogen sulfide is an important factor to determine whether air drilling technology can be used normally. The use conditions of air drilling are that the borehole wall is stable, the formation does not produce water or the water output is not large, and it does not contain hydrocarbon substances or the content of hydrocarbon substances is not high, and it does not contain H2S.
A. wellbore stability analysis. Stability analysis of formation borehole wall is one of the prerequisites of air drilling. According to the drilling geological data, the composition of formation sand, mud and shale, the timely content and cementation degree of formation sandstone are analyzed to meet the technical conditions of air drilling.
B. analysis of formation water. According to the drilled data, analyze whether there is water-out phenomenon and water-out degree in the formation.
C. formation degassing analysis. In order to ensure the smooth implementation of air drilling, it is necessary to evaluate the formation pressure and gas production of air drilling construction interval.
D.H2S formation analysis. Air drilling is mainly based on first-class well control, and second-class well control should be strengthened to prevent blowout from getting out of control. According to the experience of conventional mud drilling in China, the possibility of encountering hydrogen sulfide gas reservoir is not great, but it needs to be paid great attention to. As long as H2S is found during drilling, air drilling must be stopped.
E. determination of applicable range. Through the comprehensive analysis of wellbore stability, water production and gas production, the most suitable interval for air drilling is selected. It is required to strengthen formation monitoring during air drilling in order to find and treat water and gas reservoirs in time.
2) Determination of main parameters. There are four calculation modes of hydraulic parameters in gas drilling: Angel theoretical derivation algorithm, Ikoku et al.' s calculation method considering cuttings settlement, Adewumi et al.' s calculation method derived from fluid mechanics and Supon et al.' s experimental regression method. At present, computer software based on one or more of the above models is used to design gas drilling parameters.
Two important parameters of gas drilling are bottom hole pressure and gas flow, so it is necessary to keep enough pressure at the bottom hole to overcome the pressure drop caused by gravity and friction of suspended solid. Because the minimum buoyancy may appear at the bottom of the well and the joint between the drill collar and the drill pipe, the gas velocity must be guaranteed in these two places, and the optimal gas velocity required for gas drilling depends on the particle diameter.
In addition to the above two parameters, the parameters that affect gas drilling include ROP, cuttings size, surface atmospheric pressure, temperature and so on.
Air drilling construction in 3.3.4.4
(1) gas lift
Before air drilling, after the drill string is lowered to the bottom of the well, the drilling fluid in the well is lifted by the compressed air at the inlet of the high-pressure pipeline through the high-pressure pipeline and the drill string. Gas lift generally adopts two air compressors (displacement 54.4m3/min) and 1 supercharger (displacement 60m3/min). Control the gas injection pressure slightly higher than the liquid column pressure in the wellbore, and control the wellhead back pressure by adjusting the throttle valve to prevent the wellhead gushing from being too large or exceeding the rated pressure of the liquid-gas separator, and gradually replace the wellbore with air. After gas lift is completed, compressed air must be used to continue cleaning and drying the wellbore, and drilling can only be started after the returned gas is dried.
(2) Air drilling
In φ 3 14. 1 mm or φ3 16.5mm borehole, 4 ~ 5 air compressors are generally used for air drilling, with the displacement 100 ~ 130 m3/min and the gas injection pressure1. According to the particle size, specific gravity and wetting degree of returned cuttings, as well as the change of gas injection pressure and torque, and the blockage in the lifting process, abnormal situations such as underground water production and wellbore instability can be judged. According to different severity, measures and methods such as increasing circulation time, increasing gas injection, switching to atomized drilling, foam drilling and drilling fluid are taken respectively. Under the condition that the pressure exerted by the gas column on the underground is very small, the key is to do a good job in gas detection, hydrogen sulfide detection and well control, accurately and timely detect the change of components in the returned gas, control the underground explosion as much as possible, and prevent the occurrence of underground accidents.
(3) Trip in and out
Before gas drilling in the non-production layer, fully circulate to clean the cuttings and stop injecting gas into the well. After the compressed gas in the annulus returns, open the wellhead and trip out normally. When tripping out, pay attention to the trap pressure in the drilling tool under the check valve. When tripping in, trip in normally, connect the check valve to the top of the drilling tool and continue drilling. When gas drilling is carried out in the production zone, if there is pressure at the wellhead, it is necessary to trip by rotating the BOP, but the weight of the drilling tool cannot overcome the jacking force, so it is necessary to use the snubbing tripping device or use the downhole casing valve to trip.
(4) Replace drilling fluid
When one of the following situations is found, air drilling should be changed to conventional mud drilling:
1) formation water, the ground performance is to look at water droplets.
2) The total hydrocarbon content of the return gas continuously exceeds 3%.
3) The H2S content in the returned liquid continuously exceeds 5mg/m3.
4) Sudden increase of torque and friction or difficulty in tripping, which affects drilling safety.
5) The well deviation is greater than the design requirements, and the deviation correction effect is poor.
If the air drilling construction site is equipped with atomization and foam drilling equipment, when drilling meets formation water, it can be converted to atomization or foam drilling in time according to the size of water.
In the construction site of air drilling, the principle, method and specific implementation steps of transforming air drilling into mud drilling are formulated in detail. In the process of mud conversion, strictly implement the technical scheme of converting air drilling into mud drilling.
Application Practice of Underbalanced Drilling Technology in 3.3.4.5
(1) Application in South China
Marine gas drilling technology in southern China is mainly used in continental strata. Since air (atomization) drilling was started in the strata above the continental Xujiahe Formation, good results have been achieved, and the ROP has been increased by 5 ~ 10 times.
(2) Application in Tarim area
Since 1998, more than 60 underbalanced wells have been drilled in Ordovician carbonate strata in northern Tarim, involving production wells, appraisal wells, exploratory wells, directional wells, horizontal wells and sidetracked horizontal wells, which have solved engineering problems such as drilling leakage in Ordovician carbonate reservoirs in northern Tarim, significantly improved oil and gas production and drilling machinery speed, improved the oil and gas discovery probability of appraisal wells and exploratory wells, and improved the oil and gas discovery probability.