EP1394357A1 - Method of estimating the volume ratio of gaz to oil (GOR) in the borehole fluids while drilling - Google Patents

Abstract

The volume of gas (Vg) in the drilling fluids is the slope of a ratio measured between the gas volume produced and the corresponding volume of rock drilled. The volume of oil (Vo) is determined by measuring the proportion of organic carbon (TOC) in the drilled rock. The volume ratio (GOR) is the ratio between the determined volumes of gas and oil The volume of oil (Vo) is determined by measuring the amount of organic carbon (TOC) in the drilled rock taking into account physical characteristics of the drilled rock and of the oil in the surface conditions. The ratio (SPI) of a volume of gas produced at the surface to the same volume of rock drilled is evaluated taking into consideration the concentration of gas in the drilling fluids, the drilling fluid flows in circulation, the rate of penetration of the drilling tool and the diameter of the hole drilled. The volume ratio (GOR) is determined by the formula GOR = SPI/((TOC/100).k.(rhor/rhoo)) where rhoo is the density of the liquid hydrocarbon at the surface (generally estimated at 0.8), rhor is the estimated density of the rock, phi is the porosity of the rock and k is the ratio between % weight of oil and carbon in the rock drilled.

Description

The present invention relates to a method for estimating the volume ratio of the oil gas in the fluids of a well being drilled that tankers designate usually abbreviated GOR (for gas / oil ratio).

State of the art

A number of service companies have been measuring content for a long time mud logging (measurement of C1-C5 compounds) and some of these companies are developing measurement tools to quantify more precisely, the gas contents of the sludge, in particular for the hydrocarbons of fractions C6-C8.

In exploration as in development, when being drilled, a level rich in hydrocarbons is discovered, it is very important for the oil operator to know the type of fluids present in the formation (oil, gas, condensate, heavy oil, etc.) or even better the volume ratio of gas to oil in the fluids of a well. This information is currently accessible only through training tests that require stop drilling and represents a significant cost for the operator. With this in mind, is clear that having an approximation of this GOR during drilling would be a "plus" important for the operator because this data would help to better understand the interest of the potential deposit and would lead to a better evaluation of the sequence of drilling operation (stopping or continuing drilling, absence or triggering of a test or production tests).

In the current state of knowledge, there is no method to to know this GOR without having recourse to training tests. Only methods exist more or less empirical based on molecular ratios of gases from the drilling mud whose interpretation allows, at best, to know if the fluid present in the tank is oil, gas or heavy oils.

The method according to the invention

The method according to the invention allows during drilling to evaluate the GOR on the same drilling site by direct or indirect measurements of the volumes of gas and of oil in drill cuttings.

• la détermination du volume de gaz (Vg) contenu dans les fluides de forage par le biais d'une mesure d'un rapport entre un volume de gaz produit et un volume correspondant de roche forée ;
• la détermination du volume d'huile (Vo) par la mesure du taux de carbone organique (TOC) dans la roche forée en tenant compte de caractéristiques physiques de la roche forée et de l'huile dans les conditions de surface ; et
• la détermination du dit rapport volumétrique (GOR) en faisant le rapport des volumes de gaz et d'huile précédemment déterminés.

It essentially comprises:

• determining the volume of gas (Vg) contained in the drilling fluids by measuring a ratio of a volume of produced gas to a corresponding volume of drilled rock;
• Determining the volume of oil (Vo) by measuring the organic carbon (OC) content in the drilled rock taking into account the physical characteristics of the drilled rock and the oil in the surface conditions; and
• the determination of the said volumetric ratio (GOR) by making the ratio of gas and oil volumes previously determined.

For example, the ratio of a volume of gas produced on the surface to the same volume of rock drilled taking into account the gas concentration of the drilling fluids, the flow of drilling fluids in circulation, a penetration rate of the drilling tool and the diameter of the drilled hole.

Other features and advantages of the method according to the invention will appear more clearly on reading the description below.

DETAILED DESCRIPTION

It is recalled that the GOR (Gas / Oil ratio) is defined by the relationship GOR = V _g / V _o where V _g and V _o are respectively the volumes of gas and oil produced on the surface in standard conditions

Determination of Vg

The volume V _g of gas contained in the drilling mud is estimated from measurements of the ratio V _g / V _r (where V _r is a volume of drilled rock) provided by the operator. For example, this ratio R of the m ³ of gas produced on the surface per m ³ of drilled rock is evaluated on the basis of the following information: gas concentration of the sludge (in%), sludge flow (m ³ / min) , penetration rate of the drill bit (minute / meter) and hole diameter.

From this value R it is possible to calculate: V boy Wut = R * V r

Determination of the volume of oil Vo

We propose to estimate the volume of Vo oil using measurements made on the actual drilling site using a Rock-Eval method and analysis tool for example, from cuttings.

This process allows, in the case of a hydrocarbon deposit, to quantify the organic carbon content (OCT) present in a rock sample is described and implemented for example in the following patents of the applicant: FR 2.722.296 (US 5,843,787), FR 2,786,568, FR 2,472,754 (US 4,352,673) or US 4,153,415, concerning the Rock-Eval technique.

This organic carbon content (OCT) expressed as a% by weight of carbon per relative to the rock, allows to evaluate (considering average densities for the hydrocarbon hydrocarbon and for rock) the volumetric percentage of hydrocarbons present in a rock sample analyzed.

ρ _o est la densité de l'hydrocarbure liquide en surface (généralement estimé à 0.8) ;

ρ _r est la densité de la roche estimée en utilisant la relation ρ _r = ρ _{r 0} (1 - ) + ρ₀ . où ρ _{r 0} est la densité de la matrice (2.8 pour une dolomie, 2.71 pour un calcaire, 2.65 pour un grés) ;

 est la porosité de la roche ; et

k est le rapport entre % pondéral d'hydrocarbures et % pondéral de carbone (généralement estimé à 1.2)

Given that the sample is taken at the pressure and the temperature at the surface, it can be considered that only the liquid fraction of the reservoir fluid is present in the cuttings. The volume of oil Vo is therefore directly determined from the Rock-Eval with the relation: V o = TOC 100 . k . ρ r ρ o . V r or

ρ _o is the density of the liquid hydrocarbon at the surface (generally estimated at 0.8);

ρ _r is the estimated rock density using the relation ρ _r = ρ _{r 0} (1 - ) + ρ ₀ . where ρ _{r 0} is the density of the matrix (2.8 for a dolomite, 2.71 for a limestone, 2.65 for a sandstone);

 is the porosity of the rock; and

k is the ratio between% by weight of hydrocarbons and% by weight of carbon (generally estimated at 1.2)

Evaluation of the GOR:

It is based on the calculation of V _g / V _o : GOR = V boy Wut V o = R . V r TOC 100 . k . ρ r ρ o . V r from which it follows that: GOR = R TOC 100 . k . ρ r ρ o

Claims (3)

Method for estimating the volume ratio (GOR) of oil gas in the fluids of a well being drilled, characterized in that it comprises: determining the volume of gas (Vg) contained in the drilling fluids by measuring a ratio of a volume of produced gas to a corresponding volume of drilled rock; Determining the volume of oil (Vo) by measuring the organic carbon (OC) content in the drilled rock taking into account the physical characteristics of the drilled rock and the oil in the surface conditions; and the determination of the said volumetric ratio (GOR) by making the ratio of gas and oil volumes previously determined. Method according to Claim 1, characterized in that the ratio of a volume of gas produced on the surface to the same volume of drilled rock is evaluated taking into account the gas concentration of the drilling fluids, the flow of fluids of drilling in circulation, a penetration rate of the drill bit and the diameter of the drilled hole. Method according to claim 1, characterized in that said volumetric ratio (GOR) is determined by the relation: GOR = SPI TOC 100 . k . ρ r ρ o , where ρ _o is the density of the liquid hydrocarbon at the surface (generally estimated at 0.8);
ρ _r is the estimated rock density,  is the porosity of the rock and k is the ratio between% by weight of oil and carbon in the drilled rock.