How to develop natural gas and its principle
Natural gas, like crude oil, is also buried in underground closed geological structures, some of which are stored in the same horizon as crude oil, and some exist alone. For natural gas stored in the same horizon as crude oil, it will be mined together with crude oil. We call it a gas reservoir with only single-phase gas, and its exploitation mode is very similar to that of crude oil, but it has its particularity.
Due to the low density of natural gas, which is 0.75 ~ 0.8 kg/m3, the pressure of wellbore gas column on the bottom hole is small; Natural gas has low viscosity and low flow resistance in strata and pipelines; Because of its large expansion coefficient, its elastic properties are also large. Therefore, natural gas exploitation generally adopts self-injection method. This is basically the same as flowing oil production. However, because the gas well pressure is generally high and the natural gas is flammable and explosive, the pressure bearing capacity and sealing performance of the gas production wellhead device are much higher than that of the oil production wellhead device.
Natural gas exploitation also has its own characteristics. First of all, natural gas, like crude oil, is often a storage system with bottom water or edge water. With the development of natural gas, the elastic energy of water will drive water into gas reservoirs along the high permeability zone. In this case, due to the hydrophilicity and capillary pressure of the rock itself, the invasion of water is not the effective displacement of gas, but the sealing of undrained gas in cracks or holes, forming a dead zone. This part of the high-pressure gas trapped in the water invasion zone can be as high as 30% ~ 50% of the pore volume of the rock, thus greatly reducing the ultimate recovery of the gas reservoir. Secondly, after the water comes out of the gas well, the seepage resistance of gas flowing into the bottom hole will increase, and the total energy consumption of gas-liquid two-phase pipe flow along the oil well will increase significantly. With the increasing influence of water invasion, the gas production of gas reservoir decreases, the flow capacity of gas well weakens, and the output of single well drops rapidly until the bottom hole is seriously flooded and production is stopped. At present, the control of gas reservoir flooding mainly starts from two aspects, one is drainage, and the other is water plugging. Water plugging is to separate the gas-producing layer from the water-producing layer by mechanical plugging and chemical plugging, or to establish a water blocking barrier in the reservoir. At present, there are many drainage methods, the main principle of which is to remove the water in the wellbore. The technical term is drainage gas recovery method.
The principle of small tubing drainage gas production method is that the smaller the tubing diameter, the greater the gas velocity and the stronger the liquid carrying capacity. If the tubing diameter is selected reasonably, there will be no water at the bottom of the well. This method is suitable for gas wells with high formation pressure and low water production in the initial stage of water production.
Foam drainage gas recovery method is to add foaming agent into the well through tubing or casing. The foaming agent will dissolve in the accumulated water at the bottom of the well and react with water to form bubbles, which can not only reduce the relative density of accumulated water, but also take the water generated in the formation out of the ground with the air flow. This method is suitable for gas wells with high formation pressure and relatively low water production.
Plunger gas lift drainage gas recovery method is to lower the plunger in the tubing. When tripping in, the flow channel in the plunger is in an open state, and the plunger moves downward under the action of its own weight. When it reaches the bottom of the tubing, the flow channel in the plunger automatically closes. Because the pressure acting on the bottom of the plunger is greater than the pressure acting on the top, the plunger begins to move upward and drain the accumulated water above the plunger to the ground. When it reaches the top of the tubing, the flow channel in the plunger automatically opens again and moves down again. Through the reciprocating motion of the plunger, the accumulated liquid can be continuously discharged. This method is suitable for gas wells with sufficient formation pressure and large water production.
Deep well pump drainage gas production method is to control the bottom hole pressure by using deep well pump, sucker rod and surface pumping unit that are lowered into the well, pumping water through tubing and gas production through casing. This method is suitable for gas wells with low formation pressure, especially for water production wells in the middle and late stage, but the operation cost is relatively high.