Carbonate ROS 16-19 May 2004

Organizing Committee:

Saif Al-Arfi (ADCO)
John Hassall (ADCO)
Fathy El Wazeer (ADCO)
Asbjorn Gyllensten
(ADCO)
Khaled Al Hosani (ZADCO)
Ercan Ozer (CoreLab)
Soran Talabani (Baker)
Zaid Al Kindi (Schlumberger)

Technical Committee:

Asbjorn Gyllensten – Chairman (ADCO)
Abdulla Hamoodi – (ADNOC)
Andrew Carnegie – (Schlumberger)
Ercan Ozer – (CoreLab-Abu Dhabi)
Shehadeh Masalmeh – (Shell Abu Dhabi)
Simon Stonard – (KOC/CT-Kuwait)
Soran Talabani – (Baker Hughes

Carbonate ROS Topical Conference in Abu Dhabi

Remaining Oil Saturation, ROS, is a key input to determine the economics of oil recovery. Carbonate ROS was the topic of the three day conference held in the Beach Rotana Hotel in Abu Dhabi between 16th and 19th May 2004. The Local Chapter of the Society of Petrophysicists and Well Log Analysts (SPWLA) took up the challenging task of organizing a Topical Conference in the UAE within the first year of its inception. Our efforts were rewarded with an overwhelming participation from several major oil companies and service companies.

The Conference was attended by ninety professionals from various disciplines from all over the world, including participation from neighboring Gulf countries as well as from far away places like Alaska and Norway. The format of SPWLA Topical Conferences are off-the-record discussions, there were no papers or abstracts published.

In his opening address, Mr. Ali Al Jarwan, Deputy General Manager, ZADCO, stressed the importance of an accurate knowledge of ROS and its key impact on the optimum oil recovery.

Thirty excellent technical presentations were made during the morning sessions with working examples on different traditional methods and new techniques in determining ROS. A daily best speaker chosen by ballot on each day was presented a token memento. In the morning we had the technical presentations and after lunch we split into several smaller groups focusing on separate issues. These afternoon break-out sessions were highly informative and informal with various experts freely exchanging ideas in small focused work-groups. Each group summarized their discussions and reported back to the rest of the participants at the end of the afternoon. This arrangement encouraged lively discussion and provided a valuable learning experience for younger engineers.

This Conference was our most ambitions venture to date and we were very pleased to see that so many joined and that we were able to provide excellent logistics and quality technical content, making the Conference a resounding success! This was the first such Topical Conference in the Middle East and with Abu Dhabi at the center of some of the largest carbonate fields in the world, it was most appropriate that it was held right here!

This event would not have been possible without the active support of ADNOC and ADCO management from the inception. ADCO sponsored one of the largest Petrophysics contingents to the conference. This amply illustrates the management’s commitment towards technical excellence. ADNOC also sent a large group of staff. On the night of 17th May, National Drilling company (NDC) sponsored a memorable gastronomic and nautical experience for all the conference delegates onboard the Shuja Yacht. It was most relaxing to leisurely watch the Abu Dhabi Corniche skyline pass-by, after an intense days work.

Technical Highlights

Many techniques to estimate ROS in-situ are being used in the industry, these include:

  • Sponge core & low invasion coring
  • (Pressure core … availability ?? logistics, HSE)
  • Open Hole Logs
  • Cased Hole Logs
  • Log – Inject – Log
  • Tracer experiments

Several speakers emphasized the importance of good planning before embarking on any data acquisition programs. This is particularly important for coring, where dedicated focus on coring procedures have played a significant role in improving data quality:

  • Low invasion coring bits minimize oil-flushing
  • Mud-tracers allow accurate estimation of Swirr by comparing flushed outer plugs with virgin center plugs
  • Staged tripping reduce oil-flushing by gas expansion
  • Prevent oil-loss at surface by rapid core preservation
  • Concerns about ability of sponge to catch expelled oil

In laboratory experiments, it is important to ensure the original wettability of the core material is preserved or restored by aging with crude oil before experiments commence. It was recognized that it is not possible to represent gravity forces in the reservoir in laboratory experiments. Likewise the timing, scale & boundary conditions are very different in laboratory and the field. Also carbonate reservoir heterogeneity may not be fully captured by the limited number of coreplugs tested. Best practice would be adequate coverage of whole cores measured @ in-situ temperature, pressure and reservoir wettability state, but this is expensive and time-consuming. Centrifuge tests are more likely to reach end-point residual oil saturations; Sor.

Accuracy of log derived ROS data depends on the quality of Petrophysical parameters, resistivity index data to define the saturation exponent “n” needs to be derived from imbibition type of experiments to accurately reflect the water flooding process. Where flood water salinity differs from formation water salinity, there will always be concerns about which water salinity to use in the evaluation. Evaluating cased hole pulsed neutron capture logs depends on representative sigma-oil, -water, -matrix. It was noted that for Through Casing Resistivity logs, cement corrections become important if the flooded zone resistivity is less than 1 Ohmm – often the case in Abu Dhabi.

Estimating ROS after a gas-flood relies on the use of Neutron logs and depends on the Hydrogen Index of (HI) of the gas and oil in the reservoir. As gas passes through the oil-bearing zones, the gas will enrich itself by picking up light hydrocarbon components from the oil phase – leaving both heavier oil and heavier gas behind! The resulting changes in HIs for oil and gas will add to the uncertainty in the ROS estimate.

Error analyses should therefore focus on log uncertainty; accuracy & precision (repeatability), environmental corrections, casing and cement-effects. Some of these effects may be minimized by avoiding changes in borehole fluids and running the same logging tools for time-lapse monitoring jobs. Slow logging speeds and multiple runs may also reduce the impact of statistical noise inherent in nuclear measurements. Where possible take actual samples and measure the properties in the lab. Most critical input parameters are the matrix parameters Sma & HIma and Sgas & HIgas. Errors in Sw are significantly smaller than for Soil & Sgas.

Tilted reservoirs provide good ROS data since they allowed a prolonged time for water imbibitions of the tilted parts. Items essential for accurate ROS estimation include:

  1. Utilize cap curves to determine where the present Free Water Level (FWL) is and estimate the amount of tilt to establish if it is a transition zone or not?
  2. Distinguish between OWC variations due to tilting or reservoir quality changes.
  3. Use the correct saturation exponent “n” Archie-equation for Sw calculation: imbibition “n” in the down-tilted parts and drainage “n” for the lifted parts.
  4. Use production tests to differentiate between Residual and Remaining oil.
  5. Faults will affect water imbibitions.
  6. Integrated work between Geologist, Petrophysicist, SCAL specialist & Reservoir engineer is crucial for accurate ROS calculation from this method

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