fluid pressure
1, the definition of static pressure and static pressure gradient static pressure is the pressure generated by the gravity of the static liquid. Its size depends on the density of the liquid and the vertical height of the liquid are independent of the lateral size and shape of the liquid column. Hydrostatic pressure gradient is the amount of change in hydrostatic pressure per unit of vertical depth. The static liquid pressure gradient is affected by the density of the liquid and the salt concentration, gas concentration and temperature gradient. High salt concentration will increase the static pressure gradient, and the increase of dissolved gas volume and temperature will decrease the static pressure gradient.
2. Calculation of the hydrostatic fluid pressure
P=pgH
Where: P- -hydrostatic pressure, MPa
The p- -liquid density, and g/cm3
9- -gravity acceleration, 0.00981
H- -Vertical height of the liquid column, m
In onshore drilling operations, H is the vertical depth of the hole, the starting point starts from the disk, and the liquid density is the density of the drilling fluid.
Example 1 For a well drilled to a depth of 3000 meters, the drilling fluid density used is 1.3q / cm3, and the hydrostatic pressure at the bottom of the well is found. Solution: P=pgH=1.3x0.00981x3000=38.26MPa
3, Calculation of hydrostatic pressure gradient
According to the definition of the pressure gradient, the calculation formula is:
G=P/H=pg
Where: G 1 pressure gradient, kPa / m (MPa / m)
P-hydrostatic pressure, kPa (MPa)
Vertical height of the H-liquid column, m
P-Liquid density, g / cm)
The g-gravity acceleration, 9.81 (0.00981) With the definition of pressure gradient, the formula of static pressure can also be written as: static pressure = pressure gradient x vertical depth (P = GxH)
Example 2 A well drilled to a depth of 3600 meters, the drilling fluid density used is 1.5 g/cm3, calculate the static liquid pressure gradient in the well, solution: G=pg=1.5x9.81=14.7kPa/m
2. Equivalent drilling fluid density
1. Definition of the equivalent drilling fluid density
Equivalent drilling fluid density refers to the conversion of the sum of various pressures (static fluid pressure, return pressure, annulus pressure loss, etc.) at a certain position in the well into drilling fluid density, which is called the equivalent drilling combined fluid density at this point. The formation pressure, formation rupture pressure and circulating pressure are converted into drilling fluid density, which is called formation pressure equivalent drilling fluid density, formation rupture pressure equivalent drilling fluid density and circulating pressure equivalent drilling fluid density respectively.
2、Calculated formation loss pressure for equivalent drilling fluid density
Formation loss pressure refers to the pressure of drilling fluid loss at a certain depth. For high permeability sandstone, fracture formation and unconformity surface under normal pressure, the formation leakage pressure is often much less than the rupture pressure and is very harmful to drilling safety operation.
Cyclic pressure loss and annular air pressure loss.
Circulating pressure loss refers to the pressure loss caused by the pumping drilling fluid coming back to the ground circulation system through annular space through the ground high pressure pipe manifold, water belt, square drill pipe, underground drill column and drill spray, and other objects passing through.
Numerically equal to the drilling fluid circulating pump pressure. This pressure loss depends on the length of the drill column and the drilling fluid density, viscosity, shear force, displacement and circulation area. Pressure loss is occur when drilling fluid passes through the sink, nozzle or throttling sink. Usually, most of the pressure loss occurs when the drilling fluid passes through the drill bit nozzle. Changes in the circulation displacement can also cause large changes in the pump pressure.
The pressure loss caused by the drilling fluid is called annulus pressure loss. When the drill and pump overcome this flow resistance to push the drill and liquid to flow upward, the wall and bottom also bear this flow resistance, so the bottom pressure increases. When the drilling fluid of the stopped pump stops circulating, the flow resistance disappears, and the parallel bottom pressure returns to the static liquid pressure. The greater the return speed of the drilling fluid in the ring air, the deeper the well, the more irregular the hole, the smaller the space gap, and the higher the density and the shear force, the greater the flow resistance; the smaller the flow resistance.
5. Exciting stress and draining stress
1. Draw pressure
When the pumping pressure occurs in the well, the lifting of the drill column will lead to fill the hole space of the lower end of the drill column. This part of the drilling fluid is affected by the flow resistance, so that the internal drilling and liquid can not fill this part of the hole space in time, which forming a pumping space under the drill, and the result is to reduce the effective bottom pressure.
surge pressure
The iting pressure is generated during drilling and casing because the drill column goes downward, squeezing the drilling fluid below it to produce an upward flow. Because the flow resistance is overcome when the drilling fluid is flowing upward, the parallel wall and the parallel bottom also bear the flow resistance, making the parallel bottom pressure increases.
3. Influence factors
The iting and pumping stress are mainly influenced by the following factors:
1) Structure, size and actual length of pipe string in the well:
2) Well body structure and borehole diameter;
3) Drilling speed;
4) Drilling fluid density, viscosity and static cutting force;
5) The degree of drill or centralizer.
Therefore, when drilling the casing and casing, we should control the speed, not too fast, and more attention should be paid when drilling the high pressure oil and gas layer and drilling fluid performance is not good.
Vi. Pressure at the bottom of the well
In drilling operations, there is always pressure on the bottom of the well, mainly from the static pressure of drilling fluid. At the same time, the pump pressure used for pumping the drilling fluid along the annulus is also applied on the bottom of the well, that is, the annular pressure consumption for circulating drilling fluid. There are other pressure, excitement pressure, pumping pressure, ground return pressure of the ground fluid entering the well. The bottom pressure refers to the total pressure on the ground and in the well at the bottom of the well. In different operating conditions, the basin pressure is different.
In the static state, the bottom pressure = static pressure, the bottom pressure is mainly composed of the static pressure of drilling fluid, and the static pressure of drilling fluid is mainly affected by the density of drilling fluid and the height of the liquid column in the well. In Wells with active oil and gas, it should be noted that when the fluid in the well is static for a long time, the diffusion effect of the gas in the formation affects the fluid density in the well, which may eventually affect the bottom pressure of the well. In addition, in the static state, the parallel liquid level should be monitored to prevent the height of the liquid column from affecting the pressure of the bottom.
In normal circulation, the bottom pressure = static liquid pressure + fluid circulation in the well, the ring air pressure will increase the bottom pressure, and the excessive circulation pressure may leak; Once the circulation pressure suddenly disappears, the bottom pressure will decrease, which also affects the internal pressure balance.
During the throttling cycle, when the combined bottom pressure = static pressure + annulus pressure loss + throttle valve return throttle cycle degassing or well pressure cycle, a certain back pressure of the well head is formed by adjusting different switching degrees of the throttle valve, and the bottom pressure is maintained to balance the formation pressure.
When drilling, the bottom pressure = static pressure-pumping pressure Due to the influence of the pumping pressure, the bottom pressure of the drilling will drop, resulting in a lot of the bottom pressure can balance the formation pressure during normal drilling, and the overflow occurs during drilling. Therefore, it is necessary to judge and reduce the impact of the drawing pressure.
When drilling, the combined bottom pressure = static pressure + exciting pressure due to the excited pressure increases the bottom pressure during the drilling. Although it will not directly cause and control the problem, the excessive excited pressure may lead to leakage, resulting in the decrease of the static pressure, which leads to the well control problems. Therefore, the drilling should also do a good job of well control.
When closing the well, the bottom pressure = static pressure + ground back pressure should overflow in time to form enough ground back pressure, so that the bottom pressure can re-balance the formation pressure. The ground return pressure acts on the wellhead equipment and the whole wellbore, so it is required that the parallel port equipment should have sufficient pressure bearing capacity and sealing. The high return pressure of the ground will destroy the integrity of the wellbore, so the return pressure of the ground is not the greater, the better.
Seven, safety added value
In near equilibrium pressure drilling, the drilling fluid density is determined based on the formation pressure, and a safety added value is added to ensure the safety of operation. Because in the drilling, due to the influence of the swabbing pressure will reduce the bottom pressure, and reduce the speed of the lifting drill column and other measures can only reduce the swabbing pressure, but can not eliminate the swabbing pressure. Therefore, it is necessary to attach a safety value to the drill liquid density to offset the influence of factors such as drawing pressure on the bottom pressure. There are two main ways of addition: one is addition by density, and its safety added value is:
Oil and water well: 0.05~0.10g/cm3 gas well: 0.07~0.15g/cm32 is added by pressure, and its safety added value is:
Oil and water well: 1.5~3.5MPa Gas well: 3.0~5.0MPa
When selecting the specific safety added value, the formation pressure prediction accuracy, the depth of the formation pressure of the formation, the hydrogen sulfide content in the formation fluid, the ground stress and the formation rupture pressure, and the supporting situation of the well control device should be comprehensively considered according to the actual situation, and the reasonable selection within the specified range.
Eight, pressure difference
The difference between the well bottom pressure and the formation pressure is called the pressure difference. According to this method, the borehole pressure condition can be divided into three conditions: overbalance, underbalance and balance. Overbalance (also known as positive pressure difference) means that the bottom pressure is greater than the formation pressure; underbalance (also known as negative pressure difference) means that the well bottom pressure is less than the formation pressure; balance, the bottom pressure is equal to the formation pressure. Generally speaking, the near-balance pressure drilling refers to the overbalance pressure drilling with the differential pressure value within the specified range.
The damage of drilling fluid to oil and gas layer can not be measured solely by the density of drilling fluid, but by the size of the pressure difference and whether the chemical composition of drilling fluid filtrate matches the oil and gas layer.
Article reprint place "a little oil" public number