In fluvio-deltaic sediments of East Kalimantan, Indonesia, coal commonly occurs and poses serious problems in reservoir evaluations. Thick coal seams can easily be identified, but it is often difficult to identify and quantify coal within sandstones using the conventional well logs. This can lead to inaccurate computation of reservoir parameters such as porosity, permeability and water saturation. In this paper, it is shown how petrophysical interpretation can identify these coals and improve decision making.
In order to understand the "coaly" response on well logs, a number of core slabs were studied. It was found that the coal within the sandstones mostly occurs as thin layers interbedded with sands, not "dispersed" as often thought.Previous petrophysical analyses were reviewed where coals were not interpreted properly and resulted in over and under-estimation of coals. The new petrophysical model (ElanPlus) is based on local knowledge and extensive core data, and overcomes earlier problems. Also, the application of new logging technology (Combinable Magnetic Resonance CMR) validated the new petrophysical model and a better insight was gained.Uncertainty in quantification of coals under unfavorable situations such as, hole wash out and lack of log resolution, is now better understood.
The new petrophysical interpretation accurately quantifies coals from the conventional well logs and addresses the long standing problem in East Kalimantan sandstones. Accurate reservoir parameters can be computed and better decisions on completions can be made.
In fluvio-deltaic sandstones of East Kalimantan (Indonesia), the presence of coal is very common. Typically coals were formed by deposition of tree vegetation in swamps of an upper delta plain environment (Kutai Basin Study, 1995). Coals have two common modes of occurrence, as coal seams and as coals within the sandstones. There is one additional type that grades into shale and locally termed "Hot coal". Though coal seams cause bore hole washout and drilling related problems, they are relatively easy to identify from conventional open hole logs. However, coals within the sandstones pose serious challenge in reservoir evaluation.It is often difficult to identify and quantify coals within sandstones by using conventional open hole logs. This leads to inaccurate computation of reservoir parameters such as porosity, permeability and water saturation etc. In this paper, by integrating log and core data, reservoir evaluation problems of coaly sandstones are highlighted and practical solutions are proposed.
Coal Properties and Its Effect on Logs
Though physical properties of coals vary depending on the type of coals (anthracite, bituminous and lignite), an average property can be summarized as in table 1 (Log Interpretation Principles/Aplications, 1989; Log Interpretation Charts, 1998). As a comparison, equivalent properties of sand (quartz) and oil are also listed.
As far as conventional log responses are concerned, of most relevance are low gamma ray (GR), high neutron, low bulk density, high sonic (delta t) and high resistivity. Additionally, often there are wash outs accompanied with these coal seams. These characteristics can be observed clearly in coal seams (Fig. 1). In "Hot coal", GR is high, otherwise it shows log signature of coal seams (Fig. 2).