Pressure gradient refers to the pressure change per unit distance along the flow direction of fluid, and the common value per unit length is 100 m[1]。Available in incremental form
Also called differential pressure densimeter, it measures the pressure difference between two film pressure transmitters with a certain distance, and continuously records the pressure gradient profile of the well.Such as ignoring fluidFriction lossIt can measure the average density of the fluid in the wellbore between the depths of two sensors, and is most effective for identifying the gas position and determining the static water surface position.In a well with oil and water flowing out at the same time, when the flow rate is very high, the position of oil and water flowing out can be identified according to the density difference between the incoming fluid and the fluid already in the casing;However, when the flow rate is low, the discrimination is more complex.[3]
Application of pressure gradient in oilfield development
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Application of pressure gradient in oilfield development
Pressure gradient reactionstratumChange degree of vertical pressure, not usedReservoirThe value of pressure gradient is mainly affected by the density of formation fluid. Therefore, the application of pressure gradient can be used to judgeOil-water interface。The pressure gradient of producing reservoir is the gradient generated by injection production pressure difference, so it can be judged whether it is affected by water injection.[1]
Formation pressure gradient
The formation pressure gradient refers to the increment of pressure per unit depth of the vertical depth of the formation calculated from the ground.In the application of oilfield system, it is commonly called pressure gradient, omitting the word "formation", and the common value of unit depth is 100 meters.It should be noted that the pressure gradient is corresponding to the depth. The pressure gradient of the same well at different depths is not the same value.[1]
Start up pressure gradient
Low velocity non Darcy seepage exists in low permeability media, and the starting pressure gradient can concisely describe low velocity non Darcy seepageSeepage, YesStart up pressure gradientThe seepage movement equation of is the supplement and development of the classical Darcy law, and is the theoretical basis for the effective development of low-permeability reservoirs.The analysis shows that in the classical seepage theory, the starting pressure gradient is determined by the nature of the fluid, the surface effect of the medium and the pore structure, and the change of the seepage mode cannot affect the property of the starting pressure gradient;The method of starting flow cannot scientifically describe the phenomenon of low speed non Darcy seepage.The existence of starting pressure gradient intensifies the pressure drop, making the "pressure drop funnel" smaller and sharper;In the case of starting pressure gradient, the pressure distribution curve andOriginal formation pressureThe intersection angle increases with the increase of starting pressure gradient, and there is an outer edge of pressure disturbance.[4]
Rupture pressure gradient
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abbreviationfractureGradient is the ratio of formation fracture pressure to formation depth.The fracture gradient value used in the oilfield is calculated according to a large number of fracturing practice data in this area.The value of fracture gradient varies with different oil reservoir blocks, generally in the range of (0.15~0.18) to (0.22~0.25) 10MPa/m.The fracture pressure gradient value can also be obtained by dividing the fracture pressure calculated by the rock mechanics theory by the formation depth, but it is quite different from the actual value.The fracture pressure gradient can be used not only to estimate the fracture pressure in fracturing design, but also to judge the fracture shape.Generally, when the fracture pressure gradient of deep wells is (0.15~0.18) 10MPa/m, vertical fractures are mostly formed.[5]
Other related terms
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pressure coefficient
A dimensionless number representing the change in pressure.For example, the pressure coefficient at any point on the surface of a body or in the flow field is:
Where
Is the fluid pressure;
Free flow pressure;
Is the free flow density;
Is the free flow velocity.from
The positive and negative of can determine whether the local pressure is greater than the free flow pressure.When
>0, this area is a positive pressure area;
<0 is negative pressure area.Pressure loss in pipe flowFanOr the pressure increment of the pump can also be dimensionless into the corresponding pressure coefficient, but the specific form of defining the coefficient is slightly different.[6]
pressure drop
The pressure drop caused by the energy loss when the fluid flows in the pipe.This energy loss is caused by overcoming internal friction andturbulenceIt is caused by the collision and exchange of momentum between body points of time flow, which is manifested in the pressure difference before and after the fluid flow, that is, pressure drop.The pressure drop varies with the flow rate in the pipe.
