1. Hydrostatic pressure
1. Definition of hydrostatic pressure and hydrostatic pressure gradient
Hydrostatic pressure is the pressure produced by the gravity of a liquid at rest. Its size depends on the density of the liquid and the vertical height of the liquid, and has nothing to do with the lateral size and shape of the liquid column.
The hydrostatic pressure gradient refers to the amount of change in hydrostatic pressure per unit vertical depth increase. The hydrostatic pressure gradient is affected by the density of the liquid and the salt concentration, gas concentration and temperature gradient. A high salt concentration will increase the hydrostatic pressure gradient, and an increase in the amount of dissolved gas and an increase in temperature will reduce the hydrostatic pressure gradient.
2. Calculation of hydrostatic pressure
P=ρgH
Where: P--hydrostatic pressure, MPa
ρ--The density of liquid, g/cm3
g--acceleration of gravity, 0.00981
H--The vertical height of the liquid column, m
In land drilling operations, H is the vertical depth of the wellbore, the starting point is calculated from the turntable surface, and the density of the liquid is the density of the drilling fluid.
Example 1 A well is drilled to a depth of 3000 meters, and the drilling fluid used is 1.3g/cm3 dense. Find the hydrostatic pressure at the bottom of the well.
Solution: P=ρgH=1.3×0.00981×3000=38.26MPa
3. Calculation of hydrostatic pressure gradient
According to the definition of pressure gradient, the calculation formula is:
G=P/H=ρg
Where: G—pressure gradient, kPa/m (MPa/m)
P—hydrostatic pressure, kPa (MPa)
H—The vertical height of the liquid column, m
Ρ—density of liquid, g/cm3
g—acceleration of gravity, 9.81 (0.00981)
Using the definition of pressure gradient, the formula for hydrostatic pressure can also be written as:
Hydrostatic pressure=pressure gradient×vertical depth (P=G×H)
Example 2 A well is drilled to a depth of 3600 meters, and the density of the drilling fluid used is 1.5g/cm3 to calculate the hydrostatic pressure gradient in the well.
Solution: G=ρg=1.5×9.81=14.7kPa/m
2. Equivalent drilling fluid density
1. Definition of equivalent drilling fluid density
Equivalent drilling fluid density refers to the sum of various pressures (hydrostatic pressure, back pressure, annulus pressure loss, etc.) at a certain position in the well converted into drilling fluid density, which is called the equivalent drilling fluid density. The formation pressure, formation fracture pressure, and circulating pressure are converted into drilling fluid density, which are called formation pressure equivalent drilling fluid density, formation fracture pressure equivalent drilling fluid density, and circulating pressure equivalent drilling fluid density.
2. Calculation of equivalent drilling fluid density
Example 3 The depth of the well is 2800m, the density of the drilling fluid is 1.24g/cm3, and there is an excitation pressure of 1.76MPa acting on the bottom of the well when drilling down. The bottom hole pressure and the equivalent drilling fluid density are calculated.
Solution: bottom hole pressure P=1.24×0.00981×2800＋1.76=35.82MPa
3. Formation leakage pressure
Formation loss pressure refers to the pressure when drilling fluid is lost in a certain depth of formation. For high-permeability sandstones, fractured formations, fault fracture zones, unconformities, etc. under normal pressure, the formation loss pressure is often much smaller than the fracture pressure, and it is very harmful to the safety of drilling operations.
4. Circulating pressure loss and annulus pressure loss
Circulation pressure loss means that the pumped drilling fluid passes through the surface high pressure manifold, hose, kelly, downhole drill string, drill bit nozzle, and returns to the surface circulation system through the annular space, and other objects passing by are caused by friction Pressure loss.
The value is equal to the drilling fluid circulating pump pressure. The pressure loss depends on the length of the drill string and drilling fluid density, viscosity, shear force, displacement and flow area. Whenever drilling fluid passes through manifolds, nozzles or choke manifolds, pressure loss will occur. Generally, most of the pressure loss occurs when the drilling fluid passes through the drill nozzle. Changes in circulating displacement will also cause larger changes in pump pressure.
During the drilling process, the pressure loss generated when drilling fluid flows upward along the annulus is called annulus pressure loss. When the drilling pump overcomes this flow resistance to push the drilling fluid upward, the wall and bottom of the well also bear the flow resistance, and therefore, the bottom hole pressure increases. When the drilling fluid stops circulating, the flow resistance disappears and the bottom hole pressure returns to hydrostatic pressure. The greater the return velocity of the drilling fluid in the annulus, the deeper the well, the more irregular the borehole, the smaller the annulus gap, and the higher the drilling fluid density and shear force, the greater the annulus flow resistance; otherwise, the annulus The smaller the flow resistance.
Five, excitement pressure and swabbing pressure
1. Swab pressure
The swabbing pressure occurs when the drill is lifted in the well. As the drill string is lifted, the drilling fluid will flow downward to fill the hole space vacated by the rising of the lower end of the drill string. The flow of this part of the drilling fluid is affected by the flow resistance, so that the drilling fluid in the well cannot fill this part of the wellbore space in time. In this way, a swabbing space is formed under the drill bit, which reduces the effective bottom hole pressure.
2. Excitement pressure
The excitement pressure is generated when drilling and casing, because the drill string descends, squeezing the drilling fluid below it, causing it to flow upward. As the drilling fluid has to overcome the influence of flow resistance when it flows upwards, the wall and bottom of the well are also subjected to the flow resistance, which increases the bottom hole pressure.
3. Influencing factors
Agitation pressure and swabbing pressure are mainly affected by the following factors:
1) The structure and size of the string and the actual length of the string in the well;
2) Wellbore structure and borehole diameter;
3) Tripping speed;
4) Drilling fluid density, viscosity, static shear force;
5) The degree of mud pack of drill bit or centralizer.
Therefore, when tripping and casing running, the tripping speed should be controlled, not too fast, and more attention should be paid when drilling high-pressure oil and gas layers and the performance of the drilling fluid is not good.
Six, bottom hole pressure
In drilling operations, there is always pressure acting on the bottom of the well, mainly from the hydrostatic pressure of the drilling fluid. At the same time, the pump pressure consumed when the drilling fluid is pumped upwards along the annulus also acts on the bottom of the well, that is, the annulus pressure loss when the drilling fluid is circulated. Others include the pressure of the formation fluid that invaded the well, the activation pressure, the swabbing pressure, and the surface back pressure. Bottom hole pressure refers to the total pressure acting on the bottom of the well by various pressures on the ground and in the well. Under different operating conditions, the bottom hole pressure is different.
Static state, bottom hole pressure = hydrostatic pressure
In a static state, the bottom hole pressure is mainly composed of the hydrostatic pressure of the drilling fluid, and the hydrostatic pressure of the drilling fluid is mainly affected by the density of the drilling fluid and the height of the fluid column in the well. For 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 density of the fluid in the well, which may eventually affect the bottom hole pressure. In addition, in a static state, it is necessary to monitor the wellhead liquid level to prevent the drop in the height of the liquid column from affecting the bottom hole pressure.
In normal circulation, bottom hole pressure = hydrostatic pressure + annulus pressure loss
When the fluid in the well circulates, the annular pressure loss will increase the bottom hole pressure, and the excessive circulating pressure loss may leak; once the circulation is stopped, the sudden loss of the circulating pressure loss will cause the bottom hole pressure to drop, which also affects the pressure balance in the well.
During the throttling cycle, bottom hole pressure = hydrostatic pressure + annulus pressure loss + throttle valve back pressure
In the throttling cycle degassing or killing cycle, by adjusting the different opening and closing degrees of the throttle valve, a certain wellhead back pressure is formed, and the bottom hole pressure is maintained to balance the formation pressure.
When lifting drilling, bottom hole pressure = hydrostatic pressure-swabbing pressure
Due to the influence of the swabbing pressure, the bottom hole pressure when the drill is lifted will drop, resulting in many wells that can balance the formation pressure during normal drilling, and overflow when the drill is lifted. Therefore, it is necessary to judge in advance and pay attention to reducing the influence of swabbing pressure when lifting the drill.
When drilling down, bottom hole pressure = hydrostatic pressure + excitation pressure
Due to the generation of activation pressure, the bottom hole pressure during drilling will increase, although it will not directly cause well control problems, but excessive activation pressure may lead to leakage, resulting in a drop in hydrostatic pressure, which will cause well control problems. Therefore, well control work must also be done when drilling down.
When shutting in, bottom hole pressure = hydrostatic pressure + surface back pressure
After the overflow occurs, the well needs to be shut down in time to form sufficient ground back pressure so that the bottom hole pressure can rebalance the formation pressure. Ground back pressure acts on the wellhead equipment and the entire wellbore, so the wellhead equipment is required to have sufficient pressure-bearing capacity and tightness. Too high ground back pressure will destroy the integrity of the wellbore, so the shut-in ground back pressure is not as large as possible.
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 the operation. Because during tripping, the bottom hole pressure will be reduced due to the influence of the swabbing pressure, and measures such as reducing the speed of lifting the drill string can only reduce the swabbing pressure, but cannot eliminate the swabbing pressure. Therefore, it is necessary to add a safety value to the drilling fluid density to offset the influence of factors such as swabbing pressure on the bottom hole pressure. There are two main ways to attach:
One is to attach according to density, and its safety added value is:
Oil and water well: 0.05～0.10g/cm3 Gas well: 0.07～0.15g/cm3
The second is additional pressure, and its safety additional value is:
Oil and water well: 1.5～3.5MPa Gas well: 3.0～5.0MPa
When selecting the safety added value, the formation pressure prediction accuracy, the buried depth of the formation, the content of hydrogen sulfide in the formation fluid, the in-situ stress and the formation fracture pressure, and the well control equipment should be comprehensively considered according to the actual situation, within the specified range Reasonable choice.
Eight, pressure difference
The difference between bottom hole pressure and formation pressure is called differential pressure. According to this method, the wellbore pressure can be divided into three situations: over-balance, under-balance and balance. Overbalance (also called positive pressure difference) means that the bottom hole pressure is greater than the formation pressure; underbalance (also called negative pressure difference) means that the bottom hole pressure is less than the formation pressure; balance means that the bottom hole pressure is equal to the formation pressure. Generally speaking, near-balanced pressure drilling refers to over-balanced pressure drilling with a pressure difference within a specified range.
The damage of drilling fluid to the oil and gas formation cannot be measured solely by the density of the drilling fluid, but should be identified by the size of the pressure difference and whether the chemical composition of the drilling fluid filtrate matches the oil and gas formation.