This paper reviews the oil gas reservoir has the characteristics of low permeability, low pressure and low water saturation. Propose drilling completion cause reservoir damage is the main factor in the process of stress sensitivity and water lock.In order to prevent reservoir damage, it should first be considered the use of air drilling, second consider using circulating foam drilling fluid and oil-in-water circulating foam drilling fluid system.For horizontal well and directional well, it can considered to use glass drift beads drilling fluid system, and achieve balance pressure drilling. Eye Wells should be adopted the low density, low fluid loss and high strength cement slurry system.

Reservoir has the rich client cleat, face cleat and height growth of microcracks, which is similar to most countries in the world oil situation. Compared with the United States, China's oil reservoir has the distinct characteristic, the specific performance is on the low permeability and low pressure water saturation. 90% of the reservoir of our country permeability generally less than 3 x 10-3 um2, usually is (0.001-0.1) x 10-3 um2, which belongs to a typical low permeability, low permeability reservoir. According to rock mechanics experiment result shows that the reservoir of small young's modulus poisson ratio is big and it’s easy to deformation by external force.

Through the analysis of reservoir characteristics founded that Chinese oil reservoir is with low permeability, low pressure and low water saturation. These characteristics determine the in the process of drilling completion if you don't take effective measures, the reservoir will be badly hurt, resulting in decreased permeability, reduce production. Reservoir damage factors in the process of drilling completion basically has the following aspects:

The stress sensitive

In general, it’s rich in oil layer of the cleat, face cleat and highly development microfracture, it should have high permeability. According to the test results, however, the reservoir permeability is very low. To analyze its reason mainly is the stress sensitivity of reservoir is very strong, in the role of effective stress, some crack closure, to lower permeability significantly. Young's modulus of a large number of tests show that reservoir is far less than natural gas reservoir by almost one order of magnitude. The reservoir pore pressure under normal pressure gradient.Ordinary low solid drilling fluid density is generally higher than 1.04 g/cm3, and the density of the slurry is commonly 1.80 1.90 g/cm3. Therefore, in the process of drilling, there is pressure difference between reservoir and well cementing fluid column with inevitable, reservoir are influenced a lot by the external force. It is because of the reservoir pore pressure under normal pressure gradient, so the water in the micro cracks in the reservoir is in a state of undercompaction, its supporting ability is poor. If reservoir is in high water saturation, the porosity and crack is in filled with water, and the compressibility of water is very low, and therefore under external force, it will be supported by the water. At this point, when the water is in ultra-high pressure condition, reservoir occurs only tiny deformation, lots of closed pore and fracture will not occur. And if the water saturation in the reservoir is low, because of lack the necessary support during reservoir by compression, it cause tiny cracks in the reservoir is easy to close, which further reduce the permeability.

Water locking damage

If using the conventional drilling fluid in drilling, it’s bound to produce with fusion between liquid column and formation pressure difference, and it’s difficult for borehole wall to form mud cake and thus drilling fluid filtrate will mainly come into the reservoir, lead to damage. Reservoir micro pore can be thought of as numerous twists and turns in the bend of the capillary, and oil reservoir is usually weak hydrophilic, when foreign liquid contact reservoir, it can produce strong water absorption effect. This effect can occur even under the negative pressure condition, more intense under high-pressure difference. Due to reservoir contain low initial water saturation, the filtrate invasion in micro pore after reservoir formed in the curved liquid surface, the capillary force is produced. The energy of the reservoir will not be able to drive alone. In addition, the filtrate invasion of reservoir will increase after the reservoir water saturation, reduce the gas phase permeability, lead to serious water locking damage. The invasion of the liquid damage to the reservoir permeability is very obvious. Test showed that when the liquid saturation is of 10%, gas permeability damage was 50%, and when the liquid saturation is 30%, gas logging permeability reduced almost to zero.

To control from the drilling engineering

To prevent the harm of oil reservoir, the main measures are from the aspects of drilling engineering to a smooth trip, reduce stress, Maintain reasonable mud return rate. Try to shorten the period of reservoir of leaching time, and implement nearly balance pressure drilling.

Optimization of low density drilling fluid system

(1) Air drilling

Air as a circulation medium with drilling rate quickly, has the characteristics of the obvious effects of protecting reservoir. In our country we has adopted the air on the multi-port oil gas well drilling technology, and has obtained the remarkable effect. Oil carbon of using air hammer drilling technology have been applied in drilling. Using air drilling technology is also constrained by some conditions, such as formation must be stable, not collapsed downhole complicated conditions, such as formation cannot be out of the water at the same time. If the formation water in the process of drilling, drilling process of dust occurs when exposed to water gathered themselves together, and it’s easy to sticking.

(2) The micro circulating foam drilling fluid system

For formation water, borehole wall stability problems of oil gas well, the density of drilling fluid is needed to maintain the bottom hole pressure. In this case, the use of recycled micro foam drilling fluid technology can meet the requirements. Circulating foam drilling fluid is in ordinary low solid drilling fluid adding foaming agent and stabilizing agent dispensing system of drilling fluid, the wells within 1500m can be used successfully. The drilling fluid system is between 0.8 to 1.1 g/cm3 density is adjustable, low cost and low fluid loss quantity etc. But because the bubble is compressible fluid, and therefore not transfer MWDS in drilling of horizontal Wells and LWD signals, limiting its use.

(3) Oil-based circulating foam drilling fluid system

Oil gas reservoir in the process of drilling, sometimes encountered pore pressure is lower than 0.7 g/cm3 low formation pressure. If using water-based foam drift or hollow glass bead system, it can produce with fusion differential pressure, and make reservoir damage. For this kind of situation, oil-based circulating foam drilling fluid system is studied. The system is for diesel oil, water and three-phase foam fluid, between 0.55 to 0.9 g/cm3 density is adjustable. The system filtration quantity is almost zero, mud filtrate hard to shooting.But note that the system needs to use diesel, will have an impact to the environment, at the same time proportioning cost is higher, therefore use.

(4) High quality bentonite mud

In order to reduce the positive pressure differential and cost, water is used as the drilling fluid. In the reservoir with high formation pressure coefficient, it can reduce the pressure difference using water in the drilling process. But there will be a lot of filtrate in cementing engineering of shooting reservoir with water quality of mud cake and cause more pollution. So it need to use a moderate amount of high quality bentonite slurry, with high quality and solid phase fluid loss agent filling particles, reduce the amount of filtration and form thin and firm quality of mud cake.

(5) Low density, low fluid loss and high strength cement slurry system

High density cement slurry will produce a positive pressure differential, reservoir stress sensitivity, at the same time if high filtration quantity can also cause a lot of the filtrate of shooting reservoir. Because water mud filtrate pH value is high, it’s easy to react with reservoir after precipitation to plug hole. So it should choose low density, low fluid loss and high strength cement slurry system. At present, we have succeeded in low density cement slurry system, with reduced loss of water, can control the amount of fluid loss within the 100 ml, at the same time water mud density can be reduced to 1.20 g/cm3 and 14 mpa compressive strength.

Through the above research and analysis get the following conclusion:

(1) The main factor cause the reservoir of gas reservoir damage is stress sensitivity and water locking damage.

(2) In order to prevent reservoir damage occurs during drilling, it should strengthen the drilling engineering and technical measures.

(3) It should be reduced as far as possible between drilling fluid and reservoir pressure difference, realize nearly balance pressure drilling under the condition of meet the downhole safety.

(4) According to different reservoir optimal low density drilling fluid system, try to reduce the amount of drilling fluid filtration, reduce filtrate of shooting reservoir.

(5) In order to prevent the pollution of the reservoir in the process of cementing, it should try to reduce the density of slurry and filtration.

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