Patent EA-000175-B1 (EA)
Title: Evaluation of formation porous volume filled by hydrocarbon and method thereof
Inventor: AKKURT RIDVAN
IPC: G01V3/32
Filed: 1996-06-20
Abstract: 1. A method to estimate a formation pore volume occupied by a hydrocarbon phase or phases utilizing a NMR logging tool, the method comprising the steps of: obtaining a first pulsed NMR log of the formation, the first NMR log utilizing a pulse sequence comprising an initial 90 degree  radio frequency pulse, followed by a series of 180 degree  radio frequency pulses starting at a time period tcp1 after the initial 90 pulse, and the series of 180 degree  pulses comprising radio frequency pulses each separated by a time 2tcp,1, wherein the time tcp1i.s a time which results in the measured transverse relaxation time of the hydrocarbon phase or phases being within a range of measured transverse relaxation times that are detectable with the NMR logging tool utilized; obtaining a second pulsed NMR log of the formation, the second NMR log utilizing a pulse sequence comprising an initial 90 degree  radio frequency pulse, followed by a series of 180 degree  radio frequency pulses starting at a time period tops after the initial pulse, and the series of 180 degree  pulses comprising radio frequency pulses each separated by a time period 2tcp2, wherein the time tcp2, is a time that is different from tcp1 by an amount of time sufficient to separate resultant peaks of transverse relaxation times attributable to the hydrocarbon phase or phases within the formation; and determining from the first and the second NMR logs the formation pore volume occupied by the hydrocarbon phase or phases. 2. The method of Claim 1, wherein said hydrocarbon phase forms hydrocarbon gas, tcp1 being a time resulting in the measured transverse relaxation time of the hydrocarbon gas being within the range of measured transverse relaxation times that are detectable with the NMR logging tool utilised, tcp2 being a time that is different from tcp1 by an amount sufficient to separate resultant peaks of transverse relaxation times attributable to hydrocarbon gas within the formation, and wherein the pore volume occupied by hydrocarbon gas is determined from said logs. 3. The method of Claim 2, wherein tcp1 is a time, which is less than a time resulting in the measured transverse relaxation time of the hydrocarbon gas being greater than about 4.10<-3> seconds, and tcp2 is a time which is greater than a time resulting in the measured transverse relaxation time of the hydrocarbon gas being less than about 2.10<-3> seconds. 4. The method of any of Claims 1-3, wherein the formation pore volume occupied by the hydrocarbon phase or phases is determined by: determining, from the first and the second NMR logs, a distribution of transverse relaxation times attributable to hydrocarbon phase or phases within the formation; and determining from the distribution of transverse relaxation times attributable to hydrocarbon phase or phases within the formation the pore volume of a formation occupied by hydrocarbon gas within the formation. 5. The method of any of Claims 1-4, wherein tcp1 is less than that which results in the measured transverse relaxation time of the hydrocarbon gas being greater than about 8x10<-3> seconds. 6. The method of any of Claims 1-5 wherein tcp2 is equal to or greater than about 2.4 ms. 7. The method of any of Claims 1-6, wherein tcp1 is equal to or less than about 0.6 ms. 8. The method of any of Claims 1-7, wherein the two logs are obtained at the same time by a tool utilizing different pulse sequences in two annuli located slightly apart from each other. 9. The method of any of Claims 1-8, wherein both the first and the second log are obtained using pulse sequences wherein t delay is about six seconds or more. 10. The method of any of Claims 2-9, wherein tcp2 of the second log is sufficiently long that the measured transverse relaxation time of the hydrocarbon phase or phases is below detectable limits of the NMR logging tool. 11. The method of Claim 10, wherein the formation pore volume occupied by the hydrocarbon phase or phases is determined as a function of the difference between the integrals of the time domain responses of the two NMR logs.