basics of well testing brief contents of the course l.
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Basics of well testing – brief contents of the course. One-dimensional flow of liquid: The mass conservation law. φρ. ρ. u. ρ. u. x. +. Δ. x. x. Change of mass in volume = Flow in – Flow out per time unit.

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one dimensional flow of liquid the mass conservation law
One-dimensional flow of liquid:The mass conservation law

φρ

ρ

u

ρ

u

x

+

Δ

x

x

Change of mass in volume = Flow in – Flow out

per time unit

VolumeV: cylinder with cross-section A limited by the planes x and x + dx.

Mass density:

Flow in minus flow out:

Derivation of the differential conservation law

and tend

to zero:

Divide by

slide3
Q. Derive the mass conservation law for the tube with the variable cross-section:

Q. Using the result of the previous problem, derive the mass conservation law for the radial influx:

answers
Answers

Mass conservation in the tube of variable cross-section:

Mass conservation for the radial inflow:

slide5

General mass conservation law in three dimensions:

Q: Obtain axi symmetric mass balance equation p=p(r,t) using cylindrical co-ordinates in 3d equation:

basic idea of well testing
Basic idea of well testing
  • The well tests consist of relatively fast opening/closing wells, measuring the flowrates and following the value of the well bore pressure – non-steady state
  • Practical well testing is carried out with the help of a special software
  • The software is based on simple solutions of the flow equations

Continuity (radial inflow):

Pressure conductivity (or

Pressure diffusivity or

Piesoconductivity) equation:

Darcy:

fluid compressibility coefficient do not mix with gas compressibility z p
Fluid compressibility coefficient(do not mix with gas compressibility Z(p)!)

Q. Determine units (dimension) of fluid compressibility coefficient

Q. Evaluate the compressibility k-t of ideal gas under characteristic reservoir pressures. Do you believe that the compressibility concept is applicable to gas well testing?

Q. Will the real gas and the liquid compressibilities be higher or lower than that of ideal gas?

Order of magnitude for c = 10-4-10-5 1/bar for oil and c = 10-6 1/bar for water

slide8

in the last equation may be treated as constants

Pressure conductivity equation in terms of compressibility:

effects of fluid and rock compressibility
Effects of fluid and rock compressibility

Overburden pressure: the reservoir “keeps” the upper rock column

Porosity becomes a pressure function

Definition of formation compressibility

cf = 10-3 – 10-5 1/bar

From now on c is a total of the fluid and formation compressibilities

dimensional analysis
Dimensional analysis

Reduction of

dimensions:

slide11

Pressure diffusivity coefficient (L2/T)

Evaluation of maximum and minimum pressure diffusivity

slide12

Q. Determine dimention of D

Q. Evaluate the value of D for the cases of a) well exploitation (t0 = years) and b) well testing (hours). Use the following parameters:

Permeability a) 0.1d, b) 1md; Time t0 a) 2 years, b) 1 hour;

Compressibility factor 10-10; oil viscosity 3 cP; porosity 0.25; well radius 10cm.

answer
Answer:

On the times characteristic of reservoir development the time derivative may be neglected, and development occurs in the quasi-steady state regime. On the times characteristic of well testing the non-steady state character of the process becomes important.

three regimes of production through isolated well
Three regimes of production through isolated well

1. Spreading of perturbation around the well (hours) – to be used for well testing

B.C.

2. Uniform decrease of the reservoir pressure – semi steady-state regime (days, months)

B.C.

3. Slow decrease of the pressure drop, decrease of production (years)

B.C.

early stage constant terminal rate solution
Early stage: Constant terminal rate solution

Initial and boundary conditions:

Linearization for oil:

Self-similarity: Neither equation, nor the boundary conditions change if the following substitution is made:

Q. Check the condition of self-similarity.

boltzmann substitution
Boltzmann substitution

As a consequence of self-similarity, the solution depends on the only variable

We have:

So, the equation is transformed to the ordinary differential equation

Transformation of the initial and boundary conditions:

solution of the differential equation
Solution of the differential equation

First substitution:

First integration:

From the boundary condition:

Second integration:

exponential integral
Exponential integral

Solution for pressure in terms of the exponential integral:

Approximation of the exponential integral:

Approximate solution, including skin-factor:

slide19

Pressure diffusivity coefficient (L2/T)

Evaluation of maximum and minimum pressure diffusivity

Arrival time of the pressure wave

Evaluates: r=rw=0.1, r=10 m, r=re=333 m

slide20

Q. For times between 0 and 1 hour, and other parameters from the problem on slide 7 (with the only permeability of 0.1 Da), and for the skin factors equal to 0. and 1., build the plots of the dependence of pressure at the well versus time (use just 2 or 3 intermediate points).

Q. May skin be negative?

Q. In practical well testing, the derivative dP/dt is often used instead for P(t). Find this derivative and explain why it is so important.