Abstract

To be able to evaluate formation potentials and possible productivity, using the hydrocarbon ratios minutes after the drilling fluid attains the surface. Thereby decreasing the need for running expensive drilling operations (Drill Stern Tests, cores, log runs) when formation potential is questionable. That is to say, the use of Drill Stem Tests, cores and log analysis can be minimized through this method.

THEORY

That a knowledge of hydrocarbon ratios as they relate to formation fluids, enables a logging technician, on wellsite, to determine possible hydrocarbon shows: and that the relationship between quantitative amounts of methane (C1), ethane (C2), propane (C3), butane (C4) and pentane (C5) to in-place reservoir production and potential. Therefore, it is the presence in relative amounts, not the actual quantity of hydrocarbons in drilling fluids, that determine reservoir potential.

Introduction

In any discussion of the validity of different methods of hydrocarbon detection and evaluation, it is perhaps a good idea to try and envisage exactly what is taking place in the process of drilling a hole in the ground.

The original rock strata-reservoir or not – lies undisturbed before the advent of the bit, and Our objective is to attempt to reconstruct the exact physical properties of this strata without interrupting the drilling operation to run a D.S.T., core, etc.

As the bit approaches the strata that we are interested in, a phenomenon known as "flushing" Occurs. This, in short, is simply a replacement of some of the pore fluids of the rock by some of the mud, and is caused by the hydrostatic pressure of the mud column usually being in excess of that exerted by the formation. The amount of flushing that takes place, depends on a number of factors, e.g. (depth, ROP, hole size, volume of mud being circulated, physical properties of the formation and the mud, etc. and in fact, is very difficult to estimate. However, despite the fact that some portion of the interstitial fluids are flushed away from the well-bore, the formation will still contain some of these fluids when the bit finally does arrive.

At this point, the bit will mechanically break up the solid formation into small "cuttings" and, depending mainly on the porosity of the formation (but also on other factors) some oil and gas, if present, will be released into the mud. Most of the interstitial fluids remain contained in the cuttings, however, being released as the cuttings travel up the hole; the pressure on them being reduced from that of the hydro static head of the mud to atmospheric. Most of these fluids will be "produced" into the mud stream in the final 200 metres of travel, especially if effective permeability is present.

Some of the interstitial fluids, however will remain in the cuttings, although greate1 importance is usually placed on the returning mud stream as a source of hydrocarbon information. We have, anyway, two places in which to look for evidence of hydrocarbon in economic quantities – the mud and the cuttings.

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