Untangle Shale and Gas Effects to Estimate Porosity and Net/Gross Ratio Using a Boomerang Workflow - A Case Study in Shoreface Reservoirs in Brunei
- Chunming Xu (Production and Development Oman) | Aishah Helena Sharif (Brunei Shell Petroleum)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- Publication Date
- February 2020
- Document Type
- Journal Paper
- 112 - 127
- 2020. Society of Petrophysicists & Well Log Analysts
- 2 in the last 30 days
- 60 since 2007
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Tertiary shoreface-deltaic sediments in Brunei fields show different boomerang motifs on neutron-density and gamma ray-resistivity crossplots. A boomerang workflow named after the motifs is tested by calibrating to core data to quantify net/gross ratio and porosities under variable shale and hydrocarbon effects. The inflection between the two boomerang limbs marks the boundary between shoreface sandstones and offshore shale-type lithologies. Compared with subjective Vsh cutoffs, boomerang inflections are more objectively defendable signatures defining net and non-net rocks. Thin beds and heterolithic sandstones in lower shoreface and tidal environments are mixed in the sandstone limb near boomerang inflection. By including the thin beds and heterolithic reservoirs that are cut off by the Vsh approach, the static hydrocarbon in place in the studied fields increases by 10 to 40% based on the well data.
The shale matrix effect on porosity estimation in shaly heterolithic sandstones is resolved by interactively derived shale-line slopes without involving the uncertain clay volumes or clay parameters. Particularly, effective porosity, ϕEF, is estimated by inputting a wet-shale-line slope, ksh,, based on the shale limb on the neutron-density crossplot in each boomerang interval; it changes with depth as result of different compactions. Total porosity, ϕTOT, is estimated by a dry-shale-line slope; it is constant for most of the reservoirs based on core calibrations in the studied fields due to identical sediment provenance.
Hydrocarbon effect is independent of shale matrix effect, although they are mixed in the log responses. Hydrocarbon effect is qualitatively analyzed based on the angular rotation of the hydrocarbon-bearing sandstone limbs towards different fluid points. It is also quantitatively evaluated by the apparent fluid neutron-density parameters (φfl, ρfl) interactively determined by coherent ϕEF and ϕTOT estimations in a dual-porosity and dual-fluid model. For example, the calculated ϕTOT is significantly less than ϕEF if water parameters are used for gas-charged sandstones. By decreasing the (φfl, ρfl) from water (1, 1) until (ϕTOT – ϕEF) ≥ 0, we find the resultant ϕTOT matches with core porosity (except in unresolved thin beds); this is tested in more than 1,000 meters of cores in several fields covering a large range of lithology and hydrocarbon types. If invasion is insignificant, the resultant fluid density, ρfl, also matches with produced hydrocarbons. Therefore, the workflow not only provides coherent ϕEF and ϕTOT estimation in the rocks with variable shale and hydrocarbon effects but also the apparent fluid density profiles for hydrocarbon typing.
|File Size||16 MB||Number of Pages||16|