questions and answers after webinar.

Q1. How did you reconstruct the formation pressure by MRT?

A1. During Multiwell
Retrospective Testing (MRT) we use multiwell deconvolution to get drawdown
pressure transient response (DTR) and cross-well
pressure transient responses (CTR). DTR is a pressure impact on the central well of its unit rate production vs. time. Each CTR is a pressure impact on the central well off-set well unit rate production vs. time. To reconstruct formation pressure history, we mathematically stop central well for a user specified number of hours (usually we use start time of geological boundaries influence after the end of radial flow regime) and calculate pressure on base of off-set wells production/injection and CTRs.

Q2. What is deconvolution?

A2. Multiwell
deconvolution is a mathematical algorithm to evaluate independently pressure changes, caused by rate changes of every well at the tested area. it fits measured bottom-hole pressure history and sandface rates by DTR and CTR adjustments.

Q3. Are you using specific software or just excel?

A3. We use a specific software – PolyGon. It is used for Pressure
Transient Analysis, Rate Transient Analysis, multiwell deconvolution and 2D pressure modeling.

Q4. Is the spider plot from well 18 BHP observation effects or how it was plotted ?
A4. Yes. The “spider” plot represents well interference history. It demonstrates pressure impact on central well (#18) production and injection of each of offset wells. It is one of the results of multiwell deconvolution. It is done by CTRs convolution with each well actual production history.

Q5. Can you explain more detail about negative pressure impact?

A5. If offset well is hydrodynamically connected through the reservoir with central well its production drain pressure from the central well.  That’s why if the offset well is producer it has negative pressure impact on central well, if offset well is injector it has positive pressure impact on a central well.

Q6. So, basically majority of these cases has PDG?? or the pressure is WHP?? or calculated?
A6. Yes. MRT requires bottom-hole pressure at central well and sandface production rates of all wells at the tested area. In presented field cases central wells was oil produces and they had permanent downhole gages (PDGs). Wellhead pressure can be used for MRT only if the central well is an injector because of relatively robust BHP calculation from WHP in case of single phase water flow.

If wells don’t have PDG we can evaluate reservoir connectivity, quantify pressure interference and evaluate reservoir properties between wells by Pulse-Code Test (PCT) with PDG installation and pressure pulsation by rate changes in one of the wells. Advanced algorithms of Pulse-Code
Decomposition (PCD) made possible to reveal pressure responses without observation wells shut-in. this technology has been presented by Vladimir in previous Nafta College webinar.

Q7. Is your reconstructed pressure a unique solution or can many different combinations of properties between different wells give you the same reconstructed pressure?

A7. As an any technology, MRT has some limitations. And MRT has only three of them: - we must have bottom-hole pressure history at least at central well, we must have rate history of all the wells at the tested area and tested period must be long enough to have a lot of well rate changes, they must not be always simultaneous. If well rate changes were significant and not simultaneous MRT solution will be unique. If during production history there was not enough significant rate changes or they will be simultaneous there won’t be a unique solution. In that case we must either increase tested period of perform PCT instead of MRT.

Q8. For multizones well analysis do we have to allocate production for particular zone or we treated all zones as single flow unit?

A8. Without rete allocation at all tested wells MTR results in quantitative history of cross-well interference with tested well reconstruction without understanding thought witch layer we have an interference. This information itself can be used for pressure maintains system optimization. But if we will see some wells with the problems (actual pressure interference can be too low or too high) advanced production logging by high precision temperature and noise logging can be recommended. After that MRT will be able to provide layer by layer information.

Q9. How sensitive are the results to accuracy of GOR?
A9. If gas is an important part of total flow GOR measurements must be relatively frequent and precise for MRT analysis. Fi most of well production is oil and water, poor GOR measurements will not affect MRT a lot. One of MRT results is rate history correction.

Q10. How does commingingled injection and pressure situations complicate this understanding?
A10. The answer is the same as on question #8.

Q11. Do we have to calculate drainage area/interval before performing MRT analysis?

A11. No, one of the important MRT findings is an interference free drainage area and shape. If we will do 2D pressure modelling with DTR and CTRs from MRT we may reveal undrained arear for infill drilling to improve recovery.