RU2057904C1 - Method of borehole fixing - Google Patents

Abstract

FIELD: borehole drilling. SUBSTANCE: in agreement with method casing strings are lowered into boreholes passing through ductile rocks. Then cement mortar is pumped into interpipe space and hole clearance. Cement mortar is pumped into interpipe space from face of borehole to floor of ductile rocks and from roof of ductile rocks to hole head. Viscous-elastic fluid is pumped into interval from floor up to roof of ductile rocks. Compression coefficient β. of viscous-elastic fluid is chosen from relation given in text of description. EFFECT: increased reliability of fixing of borehole under conditions of flow of rocks without any additional expenditures for enhancement of thickness of walls of casing pipes and with due account of their strength. 2 dwg

Description

 The invention relates to the fastening of oil and gas wells in the interval of occurrence of rocks, prone to significant plastic deformation.

 A known method of fixing wells in the interval of occurrence of rocks prone to plastic deformations [1] consisting in lowering the casing strings, pumping cement into the annulus and creating back pressure on the pipes by installing a cement bridge inside the casing string. In this case, the drilling of the bridge is carried out after equalization of rock pressure on the casing.

 However, when implementing the method, errors in the time of removal of the bridge from the column are possible. In addition, often the flow rate of plastic rock is so low that it is not fixed by standard geophysical instruments and the time it takes to install a cement bridge is difficult to determine, and drilling a cement bridge in a column can lead to a violation of its tightness.

 There is also a method of fastening wells in fluid rock [2] consisting in the fact that the casing is lowered after the plastic rock fills the wellbore. The bottom of the casing string is equipped with a face mill and, having drilled the plastic core of the rock, it is rotated until the string is soldered into the rock mass.

 However, this method can be used mainly in potassium-magnesium deposits, the flow rate of which is high. In addition, it is not technologically advanced, since it is necessary to use the casing as a drill string, which can lead to an emergency. There are difficulties with the process of cementing and drilling the shoe of the column.

 The closest technical solution to the proposed one is a method of fastening wells [3], including the descent into the well of two or more casing strings and pumping cement slurry into the annular and annular spaces. At the same time, the annular space is sealed and overpressure is created in it for the period of cement solidification.

 Thus, in the well, a composite support is formed of two casing strings and cement stone between them with increased resistance to hydraulic crushing pressure.

However, under conditions of uneven loading of the composite lining with rocks due to the prevailing influence of bending moment and weak resistance of the cement stone to this type of loading, the effect of the mutual supporting effect of casing strings in such a design is significantly reduced compared to the hydraulic loading scheme, at which this effect is maximal. and is only 30-50%
EFFECT: increased reliability of fastening of wells in the presence of plastic rocks in the section by ensuring uniform strength of the lining in relation to hydraulic and uneven pressure.

, определяемым из соотношения
β

где ΔV изменение объема межтрубного пространства при деформации обсадных колонн, м³;
V_o первоначальный объем межтрубного пространства, м³;
g давление в межтрубном пространстве, МПа; при этом вязкоупругую жидкость помещают против интервала пластичных пород, а обсадные колонны в этом интервале выбирают из условия
P_нар > W_у
Р_вн. > Р_геост, где Р_нар критическое давление разрыва наружной колонны, МПа;
Р_вн. критическое давление смятия внутренней колонны, МПа;
W_y контактное давление пластичной породы на наружную обсадную колонну, МПа;
Р_геост. геостатическое давление в подошве пластичной породы, МПа.The technical result is achieved by the fact that, in the method of fixing wells in plastic rocks, including casing running into the well and pumping cement mortar into the annulus and annular space, an additional viscoelastic fluid with compressibility factor is additionally pumped into the annulus during cement cement injection

determined from the relation
β

where ΔV is the change in annular volume during casing string deformation, m ³ ;
V _{o the} initial volume of the annulus, m ³ ;
g pressure in the annulus, MPa; while a viscoelastic fluid is placed against the interval of ductile rocks, and casing strings in this interval are selected from the condition
P _nar > W _y
R _int. > P _geost , where P _nar critical pressure of the rupture of the outer column, MPa;
R _int. critical collapse pressure of the inner column, MPa;
W _y contact pressure of the plastic rock on the outer casing, MPa;
R _geost. geostatic pressure in the sole of plastic rock, MPa.

 Figure 1 presents the well, a General view; figure 2 section aa in figure 1.

 The following designations are accepted: 1 outer casing, 4 inner casing, 3 cement mortar, 2 plastic rock, 5 viscoelastic fluid.

W _{y the} contact pressure of the plastic rock on the outer casing,
P _m annular pressure
P _in hydrostatic pressure in the inner casing,
R _n hydrostatic pressure in the annulus,
Φ is the angle of coverage of pipes by rock.

 The method is as follows.

 In a well that has discovered fluid rocks (salts, plastic clays, etc.), the existing methods isolate the interval most prone to plastic flow.

 Then run the casing 1 so that its shoe is 150-200 m below the bottom of the interval of fluid rock 2. Cementing is carried out in the usual way with the injection of cement mortar 3 into the annulus.

 After the cement has hardened, the second casing 4 is lowered into the same interval and cemented so that the viscoelastic fluid 5 is in the most dangerous interval and its cement mortar is higher and lower.

β•q _→, тогда β

•

β коэффициент сжимаемости вязко-упругой жидкости,
ΔV изменение объема межтрубного пространства при деформации обсадных колонн,
V_o первоначальный объем кольцевого межтрубного пространства,
q давление в межтрубном пространстве.Viscoelastic fluid is selected as follows: the compressibility of the fluid is associated with a change in the volume of the annular space during deformation of the outer column by the dependence:

β • q _ →, then β

•

β coefficient of compressibility of a viscoelastic fluid,
ΔV is the change in annular volume during casing string deformation,
V _{o the} initial volume of the annular annular space,
q pressure in the annulus.

 From this ratio, the compressibility coefficient of the liquid is calculated for specific pipe sizes in real situations, depending on certain existing forces.

•

Состав вязкоупругой жидкости подбирается таким, чтобы прокачиваемость она теряла через 30-40 мин после закачки в опасный интервал. Нашими экспериментами доказано, что вязкоупругая жидкость в отличие от бурового раствора не фильтруется через цементный камень при реальных перепадах давления.Thus, to increase the resistance of the entire casing structure, it is necessary to choose the compressibility factor of a viscoelastic fluid
β ≥

•

The composition of the viscoelastic fluid is selected so that it will lose pumpability 30-40 minutes after injection at a dangerous interval. Our experiments have proved that a viscoelastic fluid, unlike a drilling fluid, is not filtered through a cement stone at real pressure drops.

 Filling the annular space simultaneously with the cement mortar with a viscoelastic fluid ensures, due to its limited compressibility, the flexibility of the outer column, at which hydraulic pressure develops in the end annulus, not exceeding the calculated critical crushing for the inner column and rupture of the outer.

(1) где Р средневзвешенная плотность вышележащих пород;
Н глубина залегания подошвы пластичных пород.The strength characteristics of the casing strings are determined before drilling. To do this, determine the depth of plastic rocks, which calculate the maximum geostatic pressure in the sole of plastic rocks
P _geost

(1) where P is the weighted average density of the overlying rocks;
N the depth of the soles of plastic rocks.

The resistance of the entire structure to geostatic pressure is determined by the strength characteristics of the inner casing. Therefore, the inner casing string is selected based on the condition
P _ext> P _geost.

(2) где К

коэффициент стенности колонны,
ρ- толщина стенки колонны,
D_c средний диаметр колонны,
σ_т- предел текучести стали,
ρ_н,ρ_в- соответственно наружное и внутреннее гидравлическое давление.The minimum strength of the outer casing required to withstand uneven contact load is determined by the ratio
R _nar

(2) where K

column wall coefficient,
ρ is the column wall thickness,
D _{c the} average diameter of the column,
σ _t - yield strength of steel,
ρ _n , ρ _in - respectively, the external and internal hydraulic pressure.

If the strength of the outer column does not match the contact pressure W _y , i.e. in the case of P _nar ≅W _{y the} deformation of the outer column will occur.

(3) где η* эффективная вязкость породы,
V скорость ее течения,
d диаметр трубы,
Φ- угол контакта породы с тубой.The magnitude of the contact pressure of plastic rocks on the casing is determined by the formula:
W _y

(3) where η * is the effective viscosity of the rock,
V is the speed of its flow,
d pipe diameter
Φ is the angle of contact of the rock with the tube.

η^*γ τ представляет собой напряжение сдвига, соответствующее среднему значению градиента скорости течения породы и ее средней эффективной вязкости, формулу (1) для практического использования целесообразно представить в виде:
W_y 5 τ (1-cos Φ) (4)
Деформация (податливость) наружной колонны и сжимаемость вязкоупругой жидкости в кольцевом межтрубном пространстве будут определять величину гидравлического давления (Р_в) в этом пространстве и уравновесят постепенно неравномерное контактное давление пластичной породы по мере охвата ею всего периметра наружной колонны. Собственная сопротивляемость наружной колонны сжатию без гидравлической поддержки (Р_в) практического значения иметь не может и будет выполнять роль герметичной оболочки. Прочность всей конструкции будет определяться только прочностью внутренней колонны на смятие гидравлическим давлением, возникающем в кольцевом пространстве Р_в и прочностью на разрыв этим же давлением Р_в наружной колонны.Since the quantity η ^*

η ^* γ τ is the shear stress corresponding to the average value of the gradient of the flow velocity of the rock and its average effective viscosity, it is advisable to present formula (1) for practical use in the form:
W _y 5 τ (1-cos Φ) (4)
The deformation (ductility) of the outer column and the compressibility of the viscoelastic fluid in the annular annular space will determine the hydraulic pressure (P _in ) in this space and gradually balance the uneven contact pressure of the plastic rock as it covers the entire perimeter of the outer column. The inherent resistance of the outer column to compression without hydraulic support (P _in ) cannot be of practical importance and will serve as an airtight shell. The strength of the whole structure will be determined only by the crushing strength of the inner column by hydraulic pressure arising in the annular space P _in and the tensile strength of the same pressure P _{in the} outer column.

 When the contour of the well (cavity) is completely contracted and the outer string is covered with plastic rock, the pressure on it gradually (in time comparable with the relaxation time of stresses in the rock mass cannot be accurately calculated) will rise to geostatic. Consequently, the geostatic pressure will be completely transmitted to the inner column, which is divided with the external viscoelastic fluid.

 Thus, at all stages of the interaction of the proposed lining with the surrounding plastic rocks, the inner casing will be protected from the effects of uneven rock pressure and loaded with hydraulic pressure according to the scheme most favorable for their resistance.

 The outer pipe and viscoelastic fluid in the annular space play the role of a transformer for converting uneven rock pressure into a uniform hydraulic pressure.

 Oil casing cores, usually used for fixing intervals of wells complicated by the flow of ductile rocks, in terms of strength characteristics with the corresponding internal pressure, withstand the geostatic pressure of these rocks, if applied to the pipes in a hydraulic circuit.

PRI me R. A casing string with a diameter of 244.2 mm is lowered into a well with a depth of 2400 m. At the depth of well 2203-2219, potassium and magnesium salts were discovered. When the temperature ^{of the} reservoir 50 and the mud density 1200-1360 kg / m ³ when drilling wells manifested plastic flow of the salt, the diameter of the cavity in this interval was 80 cm. The casing for covering the salt is composed of high import P-110 tubes with a wall thickness 11.19 mm in the calculation of their sufficient resistance to uniform geostatic pressure (taking into account the internal pressure of the drilling fluid) whose maximum value at a depth of 2219 m could reach ≈ 500 kg / cm ² .

120 кг/см²
Максимальную прочность труб на сопротивляемость неравномерному контактному давлению, подсчитаем по формуле (2) (без учета разницы (Р_в-Р_н) из-за ее малости)
P_нар=

≈ 56 кг/см²
Очевидно, что без "поддержки" обсадных труб в этом интервале не выдержит неравномерное давление пластичных пород, т.к.The maximum value of the non-uniform contact pressure on the casing strings in the considered interval according to the formula (3) at Φ = 50 ^о (as the most dangerous angle of rock pipe coverage) W _у =

120 kg / cm ²
The maximum pipe strength to resist uneven contact pressure is calculated by the formula (2) (excluding the difference (P _in -P _n ) due to its smallness)
P _nar =

≈ 56 kg / cm ²
Obviously, without the "support" of the casing in this interval, the uneven pressure of the ductile rocks will not withstand

R _nar << W _y
Given that after lowering and cementing the first intermediate casing string with a diameter of 244.5 mm to the design depth (2400 m) into the well, after some deepening, the next intermediate casing or production casing will be lowered, which in case of deformation by uneven pressure of the first casing in the ductile range the rocks will also not be able to resist contact pressure (which is proved similar to the previous calculation), the well fastening is used according to the considered method.

 At the same time, the first intermediate column-1 is cemented according to standard technology from its shoe (2400 m) to the bottom of the interval of occurrence of plastic rocks (2219 m), leaving them open.

 The subsequent second intermediate (or production) column is cemented so that in the annular annular space against the interval of occurrence of plastic rocks was a viscoelastic fluid (VUR), and below and above its usual cement stone. The VUR parameters are selected in such a way that it acquires the necessary viscoelastic properties and loses pumpability approximately 30–40 min after it falls into the design interval. Our experiments proved that the VUR, unlike the drilling fluid, is not filtered through cement stone at real pressure drops and ensures the joint operation of the composite lining according to the scheme considered above.

Now, if there is a VUR in the annular annular space during deformation (ductility) of the outer pipe under the influence of uneven pressure W _y (which reaches 120 kg / cm ² ), an increase in pressure will occur, which will eventually reach the mountain ≈ 500 kg / cm ² . The inner column (next intermediate or operational), designed for this pressure, will normally perceive it during the entire period of operation. An outer pipe (Р11 244.5 mm Ж11.99), which did not withstand uneven contact pressure, with a sufficient margin, resists bursting with an internal pressure P _of 500 kg / cm ² (according to reference data, the burst pressure of such pipes is ≈ 650 kg / cm ² )

 The calculated parameter of the crepe is the maximum ductility of the outer pipe, determined in each case by the actual dimensions of the annular space, and the compressibility of the VUR.

 Thus, the use of the proposed method of attachment increases the reliability of the attachment of wells in the considered difficult conditions without additional costs for increasing the thickness of the walls of the casing and taking into account the categories of their strength.

Claims (1)

METHOD FOR FIXING WELLS in plastic rocks, including casing running into the well and pumping cement mortar into the annulus, characterized in that viscous-elastic fluid is additionally pumped into the annulus with a compressibility factor

determined from the relation

where ΔV is the change in annular volume during casing string deformation, m ³ ;
V _{about the} initial volume of the annulus, m ³ ;
q pressure in the annulus, MPa,
while a viscoelastic fluid is placed against the interval of plastic rocks, and casing strings in this interval are selected from the conditions
P _{n a p} > W _y ;
P _{in n>} P _{r e a s t,}
P _{n a r} critical pressure of the rupture of the outer column, MPa;
P _{n in} the critical collapse pressure inside the column, MPa;
W _{at the} contact pressure of the plastic rock on the outer casing, MPa;
P _{r e c t of} geostatic pressures in the sole plastic rock MPa.

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