| Abstract | U plinskim bušotinama slabo propusnih ležišta dolazi do odstupanja u određivanju propusnosti stijene primjenom analize testova porasta tlaka, jer bi mjerenje trebalo trajati i nekoliko godina. Stoga je optimiranjem trajanja testa porasta, razvijena nova metodologija u kojoj su odstupanja rezultata minimalna. Novi pristup rješavanju problema je detaljnije analiziranje krivulje porasta tlaka u tzv. nepoznatom području prijelaznog perioda, temeljem trajanja testa od samo sto sati. U metodi su primijenjene nove jednadžbe za ekstrapolaciju krivulje porasta i njene derivacije do početnog ležišnog tlaka, da bi se dokazalo u kojem vremenu teoretski dolazi do pseudo-radijalnog oblika protoka. Time je omogućena prilagodba konvencionalnih metoda za analizu testa porasta, koja obuhvaća modifikacije Bourdet dijagrama i jednadžbi za određivanje propusnosti u stimuliranim bušotinama ležišta pješčenjaka i šejlova.
Nova metoda je prilagođena i za pouzdanu primjenu u programu Saphir, putem novih jednadžbi za ulazno vrijeme i protok, uz uporabu efektivnog radijusa bušotine za pojedine tipove bušotina. Tada se svaku stimuliranu bušotinu u softveru analizira ekvivalentno vertikalnoj, uz odgovarajući efektivni radijus, što bez razvoja nove metodologije nije bilo moguće. Njena svrha je jedinstvena primjena na svim testovima porasta svakog slabo propusnog ležišta, bez potrebe za dugim periodom mjerenja, jer u literaturnim izvorima nema jedinstvenog pravila za interpretaciju prekratkih ispitivanja u nekonvencionalnim ležištima. Nova metoda je uspješno primijenjena na četiri testa u hidraulički frakturiranim i višestruko frakturiranim horizontalnim bušotinama, u slabo propusnim ležištima: Eagle Ford u Texasu, Perth Basin u Australiji, Montney u Alberti i Changqing u Kini.
Odavde slijedi potvrda hipoteze istraživanja da se analizom testa porasta može pouzdano procijeniti propusnost stijene slabo propusnog ležišta. Isto tako, potvrđeno je da se konvencionalne metode za analizu testa porasta mogu prilagoditi za primjenu u nekonvencionalnim ležištima plina. Osim toga, dokazana je i hipoteza da se analizom testa porasta može minimizirati trajanje ispitivanja u bušotinama slabo propusnih ležišta. Minimalno trajanje testa porasta tlaka od samo 100 sati će, kao rezultat istraživanja, značajno utjecati i na točnost primjene proizvodnih modela. Ono će također doprinijeti i kvalitetnijoj procjeni ekonomske isplativosti cjelokupnog ciklusa proizvodnje plina, čime bi se omogućio i veći broj ispitivanja bušotina u slabo propusnim ležištima. |
| Abstract (english) | The pressure build-up test analysis of gas wells in the low permeability reservoirs shows deviation in the calculation results for rock permeability, if new methodology is not applied. The standard method of the infinite acting radial flow could not be used without the development of a new method, as the duration of the build-up test should last for up to a few years. A new method is realized by optimizing the duration of the pressure build-up test, which gives minimal deviations of the results, even for short tests, lasting approximately only 100 hours. The basic approach to solving the problem of too long a duration of the build-up test in the low permeability gas reservoirs is the more detailed interpretation of the pressure curve in each phase of the transient period at the log-log diagram. It considers a new approach for the optimal extrapolation of the build-up curve to the unknown region of the transient period, which would theoretically last until the time when the initial pressure could be reached. This research was aimed at determining the conditions under which the conventional methods in the pressure build-up test analysis, like the type curve and its derivative, can be applied to the stimulated gas wells, resulting in the acceptable deviations of the value of the rock permeability as well. A new method can be generally divided into two parts, wherefrom the first one contains six steps for the optimal extrapolation of the pressure build-up curve and its derivative, discovering the period of the pseudo-radial flow at the log-log diagram. In this manner, the condition for the application of its second part is fulfilled. It consists of the Bourdet diagram modifications for the usage in shale and tight gas sand reservoirs, based on the effective radius for different stimulated wells. In such a way the resulting match pressure from both diagrams can be calculated and used in new, modified expressions for the permeability determination in multiple fractured horizontal wells and in fractured wells. A new method is also modified for the usage in the Saphir program, on the basis of the effective radius as well, where each stimulated well is analyzed as a vertical one, with the belonging value of the effective radius. New expressions for the constant flow and its duration before the well shut-in, are introduced for the usage in the Saphir program, depending upon the modified expressions for the permeability determination in the stimulated wells. The result of the estimated permeability in multi fractured horizontal well at the Eagle Ford shale reservoir, obtained by a new methodology is 0,005 mD (5E-18 m2), while this value, determined by a new method in the Saphir program, is 0,004 mD. As both values of the rock permeability are in the range of acceptable deviations for the low permeability reservoirs, it can be concluded that a new methodology is reliable for the analysis of each pressure build-up test with the minimum duration of only 100 hours. Furthermore, the method is successfully applied to three more pressure build-up tests at different low permeability gas reservoirs. Firstly, the method was conducted for a fractured well at the Perth Basin tight gas sand reservoir in Australia, as well as for the fractured well at the Montney tight gas sand in Alberta. The fourth pressure build-up test using which the method was proven, was made at the multi fractured horizontal well at the Changqing tight gas sand reservoir in China. The estimated values of rock permeabilities in those reservoirs are very close to the original, published values, correspondingly amounting to 0,004 mD, 0,05 mD and 0,4 mD in each well. The same values determined by a new method modified for the Saphir program, are 0,004 mD, 0,06 mD and 0,14 mD. These are almost the same results in comparison to the calculated ones, based on the modified Bourdet diagram. It can also been concluded that a new method presents an appropriate solution for the permeability determination based upon the optimization of the pressure build-up test duration in each stimulated well of low permeability gas reservoirs. The duration of the pressure build-up test in the unconventional reservoirs not only affects the deviation of the analysis results of the tested wells, and the accuracy of their future production models, but is also important for the economic viability of the entire production cycle. Furthermore, due to the successful minimization of the pressure build-up test duration in low permeability gas reservoirs to only 100 hours, a higher number of well testing can be expected worldwide in the future. This is also in accordance with the accepted scenario of the International Energy Agency that the percentage of gas from unconventional gas reservoirs in the total gas production should rise from today’s 14% to 24% in 2035. A new method for the reliable permeability determination by analysis of short pressure build-up tests could also be widely used due to the presence of huge recoverable reserves of low permeability gas reservoirs amounting to 400E+12 m3, a quantity twice as large as total proven gas reserves of 200E+12 m3. |
