Well testing Pan system

Student Mr seyed Hassan zabihi shahri (3030656) MSc petroleum Engineering

Lecturer: Professor shiyi

Introduction:Well testing is disturbing the reservoir by producing from a well at a controlled flow rate (q) for a period of time (tp) and measuring the pressure response at the well bore (Pwf ) as a function of time and shut-in the same stabilized well for a certain amount of time(t) and measure the bottom hole pressure (Pws) as a function of time.These tests are called Pressure Drawdown testing and Pressure Build Up testing ,respectively ,and by applying them we can determine reservoir properties such as average permeability (k),formation damage factor (skin), initial reservoir pressure (Pi) , Drainage area (A) and so on.

-DescriptionPan system is a powerful software that helps us to calculate the mentioned reservoir properties. At the beginning by defining a simple model for reservoir and then make that more complicated we get different reservoir properties which is described later in detail We should notice that the simplest reservoir model is the best and easiest option for the start, so by assuming a radial flow in a homogeneous, isotropic reservoir and single phase fluid we start to build the model. For making our calculation easier we add two more assumption, first in outer boundary, reservoir is considered as infinite acting and secondly constant flow rate at the well bore(inner boundary)- Pressure Drawdown testing:-Semi log plot:In this report we define pressure drawdown testing and how we can get different reservoir properties in each step. The bottom hole pressure at a constant rate in an infinite acting reservoir is given by equation 1.1

1.1 The above equation shows a plot of Pwf versus log t (semi log plot) should be a straight line with slope m and intercept p at t=1 hr , when logt=0 , 1.2 According to the above equation we can get permeability-thickness (kh) from slope (m) and from intercept we can get P1hr , which by substituting in equation 1.2 we have skin factor(s)

-Log-log plot:By defining Log-log plot in pan system we are able to work on reservoir properties more deeply and precisely. In other word, in Log-log curve we can see effects of inner boundary (skin and wellbore storage) In ETR (early time region), radial flow in MTR (middle time region) and outer boundary (e.g. faults) in LTR (late time region) Note: radial flow is a straight line with the value 0.5

-Wellbore storage:When the well is first open to flow after a shut in period, the flow rate is not all from the reservoir, because of pressure depletion in well bore we have fluid expansion .by the time the amount of saved fluid in wellbore is decreasing and flow rate will become from formation .the time for well bore storage is important and by calculating that we can guess when radial flow roughly starts. In a log-log plot, a transition time of1 log cycle from wellbore storage time is usually the beginning of radial flow-Type curve matching:Is a graphical representation of the theoretical solutions to flow equations, by matching the practical value from the field with graphs in Log-log plot we get reservoir properties.in the matching process both the pressure and the pressure derivatives are plotted on a log-log graph which has dimensionless axis .By moving the derivative plateau of actual data on the dimensionless plateau of 0.5 we get the best matching and best value for permeability from MTR, wellbore coefficient (Cs) from ETRAccording to what mentioned so far we start to work with Pan System-step1.Radial flow

By drawing a straight line (0.5) in MTR we get permeability and skin factor. This straight line in Log-log plot is the straight line in semi log. Thats why we get k (from semi log slope) and s (from semi log intercept)Note: consider to set Flow model as radial homogeneous and Boundary model as infinite acting reservoir-step2.Well bore storage coefficient (Cs)

Drawing a unit slope line at the beginning of log-log curve gives us Cs (well bore storage coefficient) equal to 0.002 bbl/psi .calculating time for well bore storage is important and the theoretical formula for that is given in equation 1.3= 1.3Note: Flow model is still radial homogeneous and Boundary model is infinite acting reservoir

-step3.Boundary effects:

After MTR(radial flow region) , pressure transient reaches no flow boundaries ,it means outer boundary conditions(e.g. sealing faults or constant pressure upper or lower boundaries due to a gas cap or aquifer) appear and reservoir is not infinite acting anymore and we have pseudo steady state regime. As we know in sss flow .by drawing a unit slope line in LTR we get the Drainage area () valueNote1: The aim of late transient (LTR) analysis is to determine the most suitable fault model (e.g. single fault parallel orthogonal and so on) and get the wellbore distance from fault (L) and also Drainage area ().Note2: consider to change the Boundary model as closed system in softwareNote3: because of unit slope line in LTR we know that there is full boundary around reservoir but the shape of that (Ca) is not still clear and we get that from Cartesian plot.

-step4.Radial flow plot(semi log plot)

As we considered before ,from semi log plot we get permeability (k)from sople of straight line( ) and skin factor(s) from intercept(equation 1.2),in addition, by determining these two major reservior characteristic we can get radius of investigation(rinv),Flow efficieny(FE) and Ps(pressure drop due to skin).I just mention here some relative equations :

Flow efficiency= =Note: oil flow rate(q) is constant

Note:Viscosity(o) , compressibility(ct) and oil formation volume factor (BO)are calculated from PVT analysys

-Step 5.Cartesian plot

By choosing two points on the Cartesian plot and drawing a straight line we can get reserves (pore volume) from slope and Dietz shape factor (Ca) from interceptCalculation: , pore volume(v)=hFor calculating Ca (Dietz shape factor) we have: 1.4m: is the slope of straight line of semi log plotPint: intercept of straight line in Cartesian plotP1hr : intercept of straight line in semi log plot (Drawdown pressure after 1 hour from semi log plot)m: slope of the straight line of Cartesian plot By calculating Ca we can predict how the shape of our reservoir(drainage boundary)will be, some different shape of boundary layers are shown in Fig 1 and we see different slopes for straight lines in LTR according to the effects of boundaries(e.g faults)

Fig1.different type of boundaryFor the mentioned example in tutorial we got Ca= 31.82 which is close to 32 so we can say the well is located in the centre of a symmetrical shape-step 6: Type curve matching (auto matching)

In this step by choosing points(e.g.12) on the log-log graph we start to do type curve matching .we should add different L values(wellbore distance from boundaries) because we assume the reservoir is square and by inputting 1000 ft. for each L with a minimum to maximum(100ft-1000ft) ,software by doing around 19 iterations(try and error) will give us calculated values for L (in my example is L1=635.16ft, L2=636.12ft , L3=635.24ft, L4=635.11ft)Note: by doing this type curve matching we get more precise values for Cs (storage coefficient) and skin factor

Step 7.History match

In this step by doing a quick match we make sure ourselves that our interpretation fit the field (practical) data in drawdown test so our type curve matching gave us precise results

Discussion:

-When there is no MTR data (no radial flow in log-log plot) due to pre fracture tests in low permeability reservoir, Type curve matching is the only method to predict permeability with any degree of certainty -we should notice that in this model we assume that Cs(wellbore storage coefficient) is constant, in other word there is one single phase fluid but in majority of oil wells the tubing head pressure is below bubble point and the moment of shut in we have two phase fluid which cause non ideal storage

Conclusion:

The aim of using this exercise is to get familiar with different type of plots in well testing (semi log, log-log, type curve matching and Cartesian plot) and understand how to interpret themPa system helps us to do the calculation much easier and get the results -in the moments of matching the results of practical data with pressure derivatives plot(type curve matching)with should do that precisely to get the correct values for k and Cs ,because the dimensionless plots are close together

References:

1-Well testing analysis by, Tarek Ahmed, Paul D.Mckinney2-Modern well test analysis by,Ronald.N.Horne3- OIL well testing handbook by ,Amanat U.chaudhry4-EPS well testing