GB2439632A

GB2439632A - Method of pulse treatment for a bottom-hole formation zone

Info

Publication number: GB2439632A
Application number: GB0711648A
Authority: GB
Inventors: Dmitry Arefievich Chuprakov; Marc Thiercelin
Original assignee: Gemalto Terminals Ltd; Schlumberger Holdings Ltd
Current assignee: Gemalto Terminals Ltd; Schlumberger Holdings Ltd
Priority date: 2006-06-22
Filing date: 2007-06-15
Publication date: 2008-01-02
Anticipated expiration: 2027-06-15
Also published as: GB0711648D0; RU2320865C1; US20070295500A1; CA2590734A1; RU2006122049A; GB2439632B; MX2007007462A

Abstract

Method of treating a bottom-hole formation zone to increase well productivity and rock permeability. According to this method a pulse generator is tripped down a well and a formation pulse treatment conducted by generating negative pressure pulses of amplitude higher than the tensile strength of the formation. The method can provide the high fissuring rate by breaking formation fluid-bearing permeable rocks around a well bore.

Description

1 2439632 METHOD OF TREATING A BOTrOM-HOLE FORMATION ZONE The invention

relates to the field of oil and gas well production and can be used to treat a bottom-hole formation zone to increase well productivity and rock permeability.

At present, various methods of treating a bottom-hole formation zone are used to increase oil recovery coefficients. These include: reactant treatments of the producing formations involving the injection into a well of different processing media based on organic and non- organic substances; pulse methods combined with mechanical, thermal and chemical effects; and hydraulic fracturing of the formation.

Hydraulic fracturing, which is perhaps most commonly used, stimulates hydrocarbon production from wells through increasing the permeability of the bottom-hole zone of the producing formation by fissuring.

is The treatment of a bottom-hole zone by pressure pulses is based on elastic wave/pressure wave excitation in rock formations. The pressure wave effect was proposed more than 40 years ago as a highly efficient alternative procedure relative to other methods. However, pressure pulse methods have not yet found wide application despite beneficial results that can be realised in practice (e.g. increases in flow rate and/or oil recovery coefficient). A central problem is a lack of reliable field data and theoretical basis. Particularly, it can be impossible or difficult to predict or simulate what the effect (positive or negative) of pressure pulses on production may be.

Nevertheless, some equipment has been developed, among them surface vibrators and downhole tools (such as pressure pulse excitation tools, sparkers, and magnetostrictive and piezoceramic sources), which provide a wide range of pulse frequencies.

RU 2105874, 1998 proposes a method of treating a bottom-hole zone involving the trip of a pulse generator in a well followed by a formation pulse treatment.

An aim of the present invention is to provide a method of treating a bottom-hole zone that provides a high fissuring rate by breaking formation fluid-bearing permeable rocks around a well bore. This method can increase rock permeability through the generation of formation microfractures or the regeneration of earlier fissures, The method can be combined with hydraulic fracturing. For example, fractures can be propagated to reach the surface of hydraulic fracturing fissures, the pressure pulses forming rock pieces that break away from the fissure surface and become propants themselves.

In general, the present invention provides a method of treating a bottom-hole zone of a well in which negative pressure pulses are generated in a formation of the zone, the pulses having an amplitude which is higher than the tensile strength of the formation. The method may include providing a pulse generator in the well at the bottom-hole zone; and using the pulse generator to generate the pressure pulses.

More particularly, in one aspect, the present invention provides a method of treating a bottom-hole zone including the trip of a pulse generator down a well followed by a formation pulse treatment to generate negative pressure pulses of amplitude higher than the tensile strength of the formation.

When the method is combined with the performance of hydraulic formation fracturing, the pressure pulses can be generated as a breaking fissure grows.

Moreover, prior to pulse generation, the pressure in the bottom-hole well zone can be built up to be higher than the pore pressure in a far-field zone for the formation, or in the case of hydraulic fracturing the pressure in the created fracture can be built up to be higher than principle maximum stress in the far-field zone for the formation.

The invention can be carried out as follows.

A pulse generator is tripped in a well and negative pressure pulses are generated around an oil-bearing formation, the pulses having an amplitude which is higher than the tensile strength of the formation. A short and powerflul pulse having a magnitude of several MPa can initiate fissuring near a welibore and in a created fracture (in the case of hydraulic fracturing). Each succeeding negative pressure pulse should make formation fissures grow. In the case of hydraulic formation fracturing, the pressure pulses can be generated as a breaking fissure grows. To create ruptures prior to pulse generation, the pressure can be built up in a bottom-hole well zone such that it is higher than the pore pressure in a far-field zone for the formation, or in the case of hydraulic fracturing, the pressure can be built up in the created fracture to be higher than the principle maximum stress in the far-field zone for the formation.

As an example let us consider a straight-drilled, axisymmetric well of radius R, and a (straight and vertical) hydraulic fracture of length L in a permeable rock formation. The well cavity and the hydraulic fracture are filled with fluid at a certain pressure P. For the well Pw > and for the hydraulic fracture Pw > -a, where P0 is the pore pressure in the far-field zone (e.g. 5 MPa), and a10 is the principle maximum stress in the far-field zone (e.g. 8 MPa) (a tensile stress is taken to be positive). The pressure P, is applied for a sufficient time to build up excessive pressure in the formation (i.e. by fluid diffusion processes). Elastic motion in the fluid-bearing pore medium is described by the following equations for a medium displacement vector u and a relative fluid displacement vector w: Pü+PjGi+V[(K ()] p1#+p1+.!V [cZM (u)+M (Vi)]. (Ib)

IC

where, p is the total mass density of the saturated rock, Pi is the pore fluid mass density, G is the shear modulus, K is the bulk modulus under drainage, M is the Biot modulus, a is the elastic pore medium coefficient, p is the porosity, 7', is the rock pore tortuosity coefficient, p is the fluid viscosity, k is the rock permeability, and a superscripted point represents a time derivative. Stress components and the pore pressure are in the form of the first space derivative i and i: = 2Gev +,j((K -.G +a2M)e-ctM c). (2a) p=-aMc+M, (2h) where, _I/2lau1/a1 +a1/a), e= au,/ax1. =-aw,/ax,.

At the interface between the well fluid and the porous reservoir the following conditions are satisfied: o=-P, =O,pP (3) where, the left-hand sides of the equations are normal stress, shear stress and pore pressure, respectively, and P Pw + P(t) is the total pressure of the well fluid.

I 5 Solving equations (1 a) and (1 b) for the boundary conditions (3) at the weilbore and the hydraulic fracture gives the stress and pore pressure spatial distributions. The use of known criteria (given below) for tensile failures and failures according to a Mohr-Coulomb law provide the possibility of estimating the tensile rock failure and the failure by shear fractures: grc O +/)>T0. (4a) =ac. (4b) where, and are functions of fissure flow for ruptures and shear fractures, respectively, being used to predict rock fracturing; and TO and a are the tensile strength and the crushing strength of the rock, respectively.

The applied dynamic pulses P(t) are of negative amplitude, for example, P(t) = -P-pulse exp(-t 2 T-pulse2), where, P-pulse is the amplitude, and T-pulse is the pulse period.

Should the tensile formation strength To be about 1 MPa, the amplitude P-pulse can be rather powerful, e.g. about 5 MPa, while the T-pulse duration for a rock I 0 permeability k equal to about I 0 can be rather short, e.g. 0.01 s. Such values can produce ruptures and shear fractures around the weilbore and the created fracture.

The fissure propagation direction can be predicted by the nature of the fissures themselves, i.e. ruptures or shear fractures. With pressure reduced, the maximum tensile component is radial relative to the welibore wall and normal relative to the fissure direction at the surface of the created fracture. Therefore, ruptures propagate parallel to the weilbore boundary or created fracture. Shear fractures, if any, are inclined at an angle = ir/4 -çp/2 to the direction of principle minimum stress, where, coo is the rock friction angle.

Claims

CLAIMS

I. A method of treating a bottom-hole formation zone invoLving tripping a pulse generator in a well followed by a pulse treatment of the formation, wherein negative pressure pulses are generated by the pulse treatment, the negative pressure pulses having an amplitude which is higher than the tensile strength of the formation.

2. A method according to claim 1 wherein, prior to the pulse treatment, a pressure is built up in the bottom-hole well zone which is higher than the pore

pressure in a far-field zone for the formation.

3. A method according to claim I or 2 wherein in the case of hydraulic formation fracturing, the pressure pulses are generated as a breaking fissure grows.

4. A method according to any one of the previous claims wherein, prior to the pulse treatment, in a created fracture zone a pressure is built up which is higher than the principle maximum stress in a far-field zone for the formation.