Density of drilling fluids.

The density of the drilling fluid is a mass unit of its volume. It is expressed in kilograms per cubic meter, or compared with a mass of the same volume of fresh water at 4 °С (relative density). The density value determines the hydrostatic pressure on the bottom and the walls of the well by the washing liquid column (Ргс)

Pгc = r × g × H, Pа (5.1)

where r - is the density of washing liquid, kg/m³;

g - free fall acceleration, m/s²;

H - the height of the flushing fluid column, m

The hydrostatic pressure of the drilling fluid column must exceed reservoir (pore) pressure P_st to prevent kicks.

Reservoir fluids (oil, gas, water) exert pressure on the interstice walls. This pressure is called reservoir (pore) pressure.

There are normal (P_stⁿ ), abnormally high (P_st^ah ) and abnormally low (P_st^al) reservoir pressure. The gradient of normal reservoir pressure equals to 0.01 MPa/m, which is equivalent to a hydrostatic pressure created by the liquid column. This column has a density of 1,000 kg/m³ (fresh water column):

the deviation degree of reservoir pressure from the normal one is characterized by the coefficient of the anomalous reservoir pressure:

Kа=P_st/ P_stⁿ= P_st/1000•g•H,

For abnormally high reservoir pressure (AHRP) Kahn > 1, and for abnormally low reservoir pressure (ALRP) Kahn < 1.

Safety norms in oil and gas industry № 2.7.3.2. "The mud density at the opening of gas, oil, water containing deposits will be determined on the horizon with maximum gradient reservoir pressure in the range of compatible drilling conditions" [1].

№ 2.7.3.3. "Design choice of the mud density should provide the producing of a drilling mud column hydro - static pressure at the well bottom and it should provide the opening of productive horizon above the designed reservoir pressure on the measure not less than:

- 10% for wells with depth up to 1200 m (interval from 0 to 1200 m).

- 5% for intervals from 1200 m to the design depth.

In necessary cases the project can install the most part of mud density, but at the same time the pressure on the horizons should not exceed reservoir pressure at 15 kgf/cm² (1,5 MPa) for wells to 1200 m depth and 25-30 kgf/cm² (2,5-3,0 MPa) for deeper wells" [1].

In the safety control interests well wirings try to support the drilling fluid density at the level that is higher than it is actually necessary for the retention of fluid in the reservoir. However, it has some significant drawbacks.

First, excessive drilling fluid density can lead to such increased pressure on the bore hole walls, that the hole will collapse under the effect of tensile loadings and the drilling mud will infiltrate the reservoir through the incipient cracks.

Such destruction is called hydraulic fracturing.

№ 2.7.3.5. "The maximum permissible repression (with hydrodynamic losses) should exclude the possibility of hydraulic fracturing or drilling mud absorption at any depth intervals of compatible drilling conditions" [1]. If in the process of well drilling some acquisitions of drilling mud arise (with exit or exit without circulation), then

№ 2.7.3.6. "In such conditions well deepening should be carried out according to the plan with a complex of measures aimed at prevention of oil and gas showings. The plan should be coordinated with the regional office of State Committee for Industrial and Mining Safety Supervision of Russia and anti-gusher service". The deviation from the № 2.7.3.3 is possible also "in the design and construction of wells with the opening of the productive strata with downhole pressure close to reservoir pressure (on balance) or low than reservoir pressure (on drawdown)". № 2.7.3.7. "The deviation of the mud density (gas-free) that is in circulation more than 0.02 g/cm³ from established project value (except some cases of liquidation of gas, oil and water showings) is prohibited" [1]. When the drilling liquid column puts pressure on the bore hole wall, besides the retention of fluids in the reservoir, it helps to ensure the sustainability of the bore hole. If there is rock salt or unconsolidated clay in the reservoir rock cut, mud fluid pressure plays an important role in hole stability. № 2.7.3.5. "Intervals are composed of clay, argillite, clay shale, salt. They are proned to the loss of stability and mud fluidity. Density, filtration and chemical composition of drill mud are to make the bore hole wall more sustainable. Thus, this repression must not exceed the limits established for the interval of compatible drilling conditions. The depression on the bore hole wall is within 10-15% effective skeletal stress (the difference between the mountain and pore rocks pressure)" [1]. Geological material in the earth's crust is in a state of uniform compression (excluding tectonic forces). Geostatistical (mountain) pressure at the depth of N is equal to the pressure of the overlying rocks

Рг= rпg Н, (5.3)

where r_п - density of rocks, kg/m³. While circulating the drilling fluid puts pressure on the bottom and on the bore hole wall. This pressure consists of the hydrostatic pressure, created by the column of drilling mud and of the pressure on overcoming hydraulic resistance during its movement in the annular space (DРк.п.).

The amount of hydrostatic pressure (RGS) and pressure losses in the annular space DР_C.P.. (RCP) is called the hydrodynamic pressure (RGD). To calculate DР_C.P. RCP we use Darcy-Weisbach formula. Thus, the value of hydrodynamic pressure (RGD, PA) excluding pressure loss between the pipe joints and the bore hole walls, will be:

R_GD =R_GS+DР_C.P =r g H +S ⁿ _n+1 {li (ui2r li) / [2 (Di - dнi)]}, (5.4)

where n - is the number of annular space intervals with permanent gap between the pipes and the bore hole walls;

li - coefficient of hydraulic resistance at the drilling mud movement in the i - th annular space interval;

ui - drilling mud flow speed in i - th annular space interval, m/s;

li- the length of the i - th annular space interval with a constant gap between the pipes and the bore hole wall, m;

Di - bore hole diameter on i - m interval, m;

dнi - outer pipe diameter on i - m bore hole interval, m;

It is obviously necessary that hydrodynamic pressure was less than hydraulic fracturing pressure (RGD < RGR) for the prevention of hydraulic fracturing and absorption. Only the hydrodynamic pressure is regulated (controlled). As it appears from the formula (4), the reduction in the value of the hydrodynamic pressure can be due to the reduction of density, viscosity, mud flow rate and the increasing gap between the drill pipes and the bore hole walls. Secondly, increasing the density of the drilling fluid gives a negative effect on the penetration rate. With the growth of the hydrodynamic pressure on the well bottom the mechanical speed is significantly reduced. This is explained by the fact that the separation terms and downhole particles of drilling cuttings displacement deteriorate, because there is an increase of pressure difference, and it presses particles of drilling cuttings down to the bottom. Particles of cuttings are held on the bottom by forces, caused by the difference between the hydrodynamic pressure on the bottom and pore pressure in the reservoir. This pressure is called differential pressure (EP)

K_MG = R_GD - R_P (5.5)

There are three ways of reducing the power pressing particle drilling cuttings to the bottom:

- reduction of the particles surface area, where there is a differential pressure;

- reduction of hydrodynamic pressure (lower density, viscosity and flow rate of drilling mud, as well as increasing gap between the drill pipes and the bore hole walls);

- the increase reservoir (pore) pressure at the depth of formation destruction to the value of the hydrodynamic pressure. That is possible with high mud spurt.

In the third, the increase of density of drilling fluids requires additional expenses for their weighting. They are entering a special weights, preparation and cleaning out (additional equipment), and also the condition maintenance (chemical reagents treatment). The cost of the mud increases. During drilling reservoirs with normal pressure the cost of drilling fluid is not so important, because the sufficient density is achieved automatically thanks to the solid phase, dispersible by the mud of the pervaded reservoirs. It is impossible to support the density of the drilling fluid at a level above 1320 kg/m³ by dispersible solid phase because of too significant viscosity increase. In such conditions it is expedient to increase the weight of washing fluid with barite, whose density is 1.5 times higher than the density of geological material. That is why significantly less solid phase is required for any given density of the drilling fluid. Consequently, the drilling mud density must be such that it would be possible to ensure sufficient pressure on pervaded reservoirs together with other technological factors and methods, but at the same time, it should not considerably worsen the working conditions of the drilling bit and operational characteristics of productive horizons. In other words, in each case the optimal value of the mud density should be selected. The specific gravity hydrometer ADB-1, the beam balance-densiometer GRP-1, pycnometer, densiometers WUA-1, PP-1, density indicator or their foreign equivalents are used for measurement of drilling fluids density.

Дата добавления: 2015-11-16