US 4,393,486 A · Method for interpreting well log records to yield indications of gas/oil in an earth formation such as a sandstone, limestone, or dolostone

Abstract

The present invention indicates that acoustic characteristics associated with a first well penetrating a gas/oil-containing strata such as a sandstone, limestone or dolostone, can be normalized (and favorably compared) with similar characteristics of a second, contiguous well of known hydrocarbon potential, such characteristics being calculated and displayed by means of a machine-implemented data processing method in which well logging and geologic data are fed thereto to calculate such characteristics without the need for shear-wave velocities. In more detail, in accordance with the invention brine-saturated bulk and shear moduli, (i.e., Kw* and Gw*) of a sandstone, limestone or dolostone penetrated by the second well can be predicted as a function of, say, brine-saturated P-wave modulus (Pw*) alone (independent of shear-wave velocity). In that way, resulting acoustic values including distinct velocity and amplitude values as a function of a saturation operator can ultimately be provided. Such values, when compared to actual, in-well, field-generated characteristics of the first well, are surprisingly accurate predictors of the amount of gas/oil saturation in and around the first well. The method has particular accuracy in designating gas zones within formations of interest due to use of the complete Benedict-Webb-Rubin non-ideal gas law in pseudo-reduced form.

Claims12

Dependent claims

Method for resolving events of an acoustic log of a first well associated with at least a selected zone of an earth formation so as to determine gas/oil fractions therein by creating synthetic gas/oil-indicating characteristics from brine-saturated petrophysical parameters corresponding to said selected zone of interest at a second well, including P-wave velocity, bulk density and porosity, with or without mineralogical inputs, as a function of gas/oil saturation, temperature and pressure (depth), but without shear-wave velocity inputs, comprising the steps of:

(a) selecting a lithology for said second well at said selected zone of interest from a group comprising sandstone, limestone and dolostone:
(b) from brine-saturated bulk and shear moduli (Kw* and Gw*) calculated from empirical equations including true brine-satuated P-wave values (Pw*) of a sample which corresponds to said zone of interest based on well logging or geologic data, and estimated frame modulus values (Ka) of said zone of interest, predicting a plurality of bulk moduli (K*) with gas/oil present in said zone of interest in preselected amounts:
(c) predicting a multiplicity of P-wave moduli of said zone with gas/oil present (Pgas/oil*) in said preselected amounts based on the relationship
P.sub.gas/oil *=K.sub.gas/oil *+(4/3)G.sub.w *
where Pgas/oil* is a multiplicity of predicted P-wave moduli with gas/oil present in said preselected amounts;
Kgas/oil* is a plurality of predicted bulk moduli of said zone with gas/oil present in said preselected amounts;
Gw* is the brine-saturated shear modulus of said zone;
(d) predicting from acoustic velocities within said zone calculated from step (c) as a function of different gas/oil gravities and saturations, synthetic acoustic values based on a relationship involving variation of said values as a function of different gas/oil gravities and saturations;
and
(e) comparing said synthetic values indexed to different gas/oil gravities and/or saturations with actual field-produced values produced from said acoustic log to predict amounts of gas/oil in said zone of interest.

7 more elements

System for resolving event characteristics of an acoustic log of a first well so as to determine gas/oil fractions in a selected zone of an earth formation by creating synthetic gas-indicating characteristics from brine-saturated petrophysical parameters corresponding to said selected zone of interest associated with a second well, including P-wave velocity, bulk density and porosity, with or without mineralogical inputs, as a function of computed values of a gas/oil saturation, temperature and pressure (depth) but without shearwave velocity input, comprising:

(A) a digital programmable computer for:
(i) selecting a lithology for said selected zone of interest of said second well from a group comprising sandstone, limestone and dolestone;
(ii) predicting brine-saturated bulk and shear moduli (Kw* and Gw*) using empirical equations containing coefficients and P-wave modulus values (Pw*), said coefficients varying in empirical fashion as a function of at least porosity, pressure and temperature, said P-wave modulus value (Pw*) being a true P-wave modulus value for a brine-saturated sample which corresponds to said zone of interest based on well logging or geologic data;
(iii) estimating frame modulus values (Ka) of said zone of interest using a relationship involving Kw*, Kb, Km and .phi., where Kw* is brine-saturated bulk density of the aggregate, Kb is the bulk modulus of brine, and Km is the bulk modulus of the solid material making up the zone of interest and .phi. is porosity;
(iv) predicting bulk modulus K* with the gas/oil present in said zone of interest using a relationship involving Ka of step (ii), Km, .phi., Kf, where Kf is the bulk modulus of a liquid-gas mixture of a selected oil/gas gravity and saturation;
(v) assuming Gw* is equal to Ggas/oil*, predicting P-wave modulus of the zone with gas/oil present (Pgas/oil*) based on the relationship
Pgas/oil/oil*=Kgas*+(4/3)Gw*
(vi) predicting at least acoustic velocities within said zone at said second well from said P-wave modulus values (Pgas/oil*) of step (v) as a function of said selected gas/oil gravity and saturation;
and
(B) comparitor means connected to (A) for comparing said predicted velocities or derivations thereof with actual velocities or derivations thereof, from said acoustic log of said first well to predict presence and amounts of gas/oil fractions in said zone of interest at said first well.

7 more elements

System for resolving event characteristics of a sonic log of a first well so as to determine gas/oil fractions in a selected zone of an earth formation by creating synthetic gas/oil-indicating characteristics from brine-saturated petro-physical parameters corresponding to said selected zone of interest but associated with a second well, as a function of computed values of gas/oil saturation, temperature and pressure (depth), but without shear-wave velocity inputs, comprising:

(A) a programmable digital computer for:
(i) selecting a lithology for said selected zone of interest of said first and second wells from a group comprising sandstone, limestone and dolostone;
(ii) from calculated normalized brine-saturated petro-physical parameters involving true laboratory and/or well-log generated brine-saturated P-wave modulus values for a brine-saturated rock sample corresponding to said zone of interest, predicting compressional velocities of said zone with gas/oil in differing amounts present in said zone, in accordance with the relationship ##STR3## where rho-B is the bulk density, .phi. is the porosity, G* is the shear modulus at the saturations of interest, Kf equals the bulk modulus for the liquid-gas mixture, Ka is the frame modulus Km is the bulk modulus of a hypothetical rock, i.e., same composition but perfectly cemented with zero porosity;
(iii) from sonic velocities of said zone calculated from (ii), predicting synthetic sonic information with gas/oil present in said zone;
and
(iv) comparing said predicted sonic information with gas/oil present with actual field-acquired logging data of said first well so as to determine both presence and amounts of gas/oil within said zone of interest at said first well.

3 more elements

Dependencies are inferred from the claim text.

Method for interpreting well log records to yield indications of gas/oil in an earth formation such as a sandstone, limestone, or dolostone

Life of this patent

Method for interpreting well log records to yield indications of gas/oil in an earth formation such as a sandstone, limestone, or dolostone

Life of this patent

Abstract

Claims12