Method and tool for evaluating a geological formation

Abstract

A method of evaluating a geological formation (20) of non-homogeneous porosity and permeability, particularly in exploration for hydrocarbon fluids, comprising the steps of:(a) inserting a formation evaluation tool (30) into a borehole (10) to a location (10a) within the geological formation (20);(b) setting the formation evaluation tool (30) into position isolating interval(s) of the borehole (10) with a packer arrangement (50) forming portion of the formation evaluation tool (30), for sample evaluation for said interval(s);(c) extracting representative samples of formation fluid from the location within the geological formation (20) for evaluation under downhole conditions using sampling means (32a, 52b, 54b) operative to extract formation fluid samples over a range of porosity and permeability encountered within the geological formation (20); and(d) analysing the formation fluid samples and measuring formation (20) permeability.A formation evaluation tool (30) for practising the method, and including an packer arrangement (50) for isolating intervals of the borehole (10) for sample acquisition and evaluation, is also disclosed. The method and tool (30) are particularly suited to geological formations comprising complex carbonate rocks such as mudstones, Wackestone, Packstone, Boundstone and crystalline carbonate rocks which typically include features such as vugs and fractures.

Description

FIELD OF THE INVENTION[0001]This invention relates to a method and tool for evaluating a geological formation, particularly a formation containing hydrocarbon fluids.BACKGROUND TO THE INVENTION[0002]Oil and gas exploration involves investigation of geological formations for locating hydrocarbon reserves fit for commercial exploitation. Such geological formations may be termed reservoir formations. Oil and gas explorers are constantly trying to find better, more reliable, faster and cheaper ways to assess the potential of prospective reservoir formations as exploration costs can range into the tens and hundreds of millions of US dollars. This is a significant challenge as reservoir geology may be highly complex.[0003]To assist this process, the Applicant has developed a formation evaluation device or tool suitable for evaluating geological formations containing hydrocarbon reservoirs. Such geological formations tend to be porous and permeable, the pores containing hydrocarbon fluids ...

AI Technical Summary

Benefits of technology

The present invention provides a way to evaluate non-homogeneous porosity and permeability in a geological formation, especially those that have complex carbonates such as vugs and fractures. The invention aims to make the process easier and more efficient.

Problems solved by technology

This is a significant challenge as reservoir geology may be highly complex.

Carbonates are rigid, especially when well cemented, and thus readily fracture when folded.

Such rocks are thus often over pressured.

Evaluation of formations containing the complex carbonate rock types is challenged by the typical considerable variation or heterogeneity in rock porosity and permeability caused by the above phenomena.

Such variation may occur even over a very short interval, or within a relatively small region, within a geological formation causing, during formation evaluation, risk of over or under-estimating hydrocarbon reserves.

In addition, non-homogeneous and large pore size, fractures and vugs are likely to cause significant difficulties with sealing of conventional sample pads against borehole walls and this affects formation evaluation as well.

In fact, sealing may not be possible.

Such sample pad designs which depend on direct sealing between sample pad, having a sample port formed in the surface, and borehole wall are not effective for extracting formation fluid samples from formations including fractures and vugs.

Images