STILL on LOW RESISTIVITY PAY…..
Report on FOSI Open Discussion on Low Resistivity Pay Part 2
(as published in Berita IAGI No. 09.38 May 2003
by Nila Murti
The problem of Low Contrast Pay is apparently quite often faced by many geoscientists working in Indonesian region. Therefore its installment for the second time as FOSI Open Discussion theme still holds the interest of many. The forum was held in Tamnak Thai restaurant in Menteng, Jakarta on Friday evening 25th April 2003, and it was attended by around 25 people from both oil and service companies. It is the second forum held this year by FOSI (Forum Sedimentologiwan Indonesia or Indonesian Sedimentologists Forum, a commission of IAGI). The first one was held early February in the same place, on the same petrophysical theme, which due to its popularity was deemed appropriate to be discussed in the forum a second time.
The first speaker was Hasto Widodo from Lemigas, who was asked to share his experiences in petrophysics as a substitute speaker on a very short notice. His experience on the subject made the short notice not a problem. Hasto shared his thoughts on the alternative method to determine water saturation. Hasto began by explaining the basic Archie equation, and the principle meaning of F in the equation, which is a very important concept. It is important because F can be drawn from just one log type (resistivity), and yet from it people can infer the property of a rock. However later it was found that F value is not always constant and as salinity rise, so does F value. Hasto thought that the main failure in Archie’s concept is because he considered Rw as remaining constant after water is hold in the rock. According to Hasto water would precipitate or dissolve minerals depending on its ion concentration, temperature and pressure, and this changing mineral content would therefore change Rw. It is therefore Rw which has to be corrected, instead of Ro as is generally done today. To overcome the difficulties of correcting Rw, Hasto proposed an alternative method for determining Sw, which is based on the following formula:
Sw = (log(Rtread) – log(RTmax)) / (log(RTmin) – log(RTmax)).
The equations of log(RTmin) and log(RTmax) which are respectively the line equation of water saturated rock and oil saturated rock, are determined by plotting the resistivity values in a logarithmic scale against Vsh and determine the lines for water saturated and oil saturated rocks. He closed the presentation by inviting people to test the equation on their study areas.
A study case of Low Contrast Pay from Central Sumatra was presented by Tunggal and Bambang Widarsono from Lemigas. According to Tunggal, many LCP cases have been reported from areas in Indonesia and these cases are quite similar, although the cause can be quite different. The consequences of LCP are mis-interpretation of fluid saturation, difficulties in delineating hydrocarbon boundaries, overlooked/bypassed productive zones and under-estimation of reserves. Tunggal mentioned several factors causing LCP, including the presence of clay minerals or iron associated minerals, the presence of thin laminated sand-shale sequence and the presence of micro-porosity. So far there is no single solution to solve LCP problems, and each case has to be solved using an integrated approach to characterise the reservoir. Data such as core and DST have to be integrated with log data to develop a complete understanding of the LCP interval. Tunggal and Bambang proposed the following procedures to deal with the LCP problems:
- Identification of clay type and its content, using spectral gamma-ray, petrography on core or cuttings (thin section, SEM, XRD, EDAX) and determination of cation exchange capacity of rock sample.
- Usage of high resolution logging tools or imaging tools to evaluate thin lamination
- Identification of micro porosity through petrography analysis and capillary mercury injection, measurement of both total and effective porosity through routine core analysis, determination of the relationship between total and effective porosity for each reservoir facies (fuzzy logic and neural network techniques can be implemented here), followed by normalisation of the calculated Sw within pay zones by eliminating the effect of micro porosity.
- Determination of the volume of iron associated mineral using the elemental analysis approach, correction of resistivity log by implementation of series or parallel circuit formula, recalculation of Sw using the corrected resistivity log and validation of the log-derived Sw against the Sw derived from integrated formation test and core analysis (relative permeability or capillary pressure data).
This procedure was implemented in their case study, in which the low resistivity pay zones are thought to be caused by the abundant presence of glauconite. In this case study the log-derived Sw match the Sw derived from integrated formation test and capillary pressure data in the normal resistivity pay zones, thus confirming the calculation. However in the low resistivity pay zones the Sw derived from integrated formation test and capillary pressure data proved to be more accurate in determining the water saturation. In these zones the Sw from capillary pressure showed lower values and therefore indicating pay.
The last presentation for the night was given by Mimbar Bambang Seputro from Geoprolog which has kindly sponsor the event. Mimbar showed Geoservices’ Reserval tool, which is an advanced FID chromatograph and advanced gas trap tool, which can detect drilling gases more accurately even in fast drilling. In FOSI Open Discussion held in early February a participant showed a case study in which a low contrast pay zone was identified during drilling using this tool.
After the presentation, while enjoying the refreshment the discussion was continued on the origin and sedimentological implication of the presence of glauconite. The presence of glauconite in the case study presented by Tunggal was thought to be indication that the sediments were of shallow marine origin. A participant with some knowledge of Central Sumatra area proposed that the formation be of estuarine environment. Another participant reminded the forum to be more careful in making the connection between sedimentary environment and the presence of glauconite, since minerals are not only the result of primary sedimentation but also can be resulted from diagenesis.
Before the discussion was closed at 7.45 pm, the moderator throw one last question to the participants, requesting them to forward any idea, topic or themes for the next open discussion forum. To forward an idea, please contact Nila Murti or FOSI’s General Secretary Hasan Sidi.
PHOTOGRAPHS TAKEN IN THIS EVENT
Hasto Widodo from Lemigas proposing an alternative method for Sw calculation and inviting participants to try the equation in their projects.
Participants give thoughts, ask questions and forward comments, even share their experiences and study cases, making the discussion in FOSI Open Discussion lively
Mimbar from Geoprolog giving a presentation of Reserval, an advanced FID chromatograph and gas trap tool.
Tunggal from Lemigas proposing a procedure to approach Low Contrast Pay cases and presenting a case study of Low Resistivity Pay from Central Sumatra area.
Participants seriously sharing their experiences in FOSI Open Discussion.
The informal atmosphere of a FOSI Open Discussion forum.
The material for discussion in a FOSI Open Discussion forum is presented in an informal setting. Questions and discussion thoughts are encouraged from participants even as the material is presented. Refreshment can also be onjoyed freely amid the discussion.
On behalf of FOSI Nila Murti the moderator of the discussion formally thank Geoprolog representative Mimbar Bambang Seputro for the sponsorship of FOSI Open Discussion II, 25 April 2003 in Tamnak Thai Restaurant Jakarta.
