Application of new viscoelastic acidizing of diverting technology in horizontal wells of Tarim oilfield

Abstract

The Donghe block of Tarim oilfield is a long horizontal well sandstone reservoir with lots of small layers and serious heterogeneity. After conventional acidizing, a small increase in oil production, but an excessive increase in water production, was observed. Through reason analysis, the main issue was that in acidizing engineering, the acid fluid entered the high-permeability zone preferentially, the low permeability without acid fluid, and the interlayer heterogeneity was intensified. In view of the problems such as the difficulty of uniform distribution acid and the unsatisfactory effect during the acidizing, the acidizing of diverting-steering technology was proposed. On the one hand, the mechanism of viscous diverting was analyzed; on the other hand, the concentration of viscoelastic surfactant DCA-1 was optimized, which is suitable for the diverting of sandstone reservoir acid, and its diversion performance indoors was evaluated. Before acidizing, the production data were 1.64 t/day oil and 1.71 t/day water, with water content of 51%. After acidizing, the production data were converted into 18 t/day oil and 0.0 t/day water, with water content of 0%. The effect of acidizing is obvious. Field application showed that acidizing diverting technology has achieved a good reservoir stimulation effect in Donghe block, so it has a reference significance for the stimulation of sandstone reservoir acidizing uniformity in a long well section.

Appendix. Mechanism of action of the viscoelastic diversion acid system

In order to analyze the diversion and solution of viscous liquid, the following assumptions are made: (1) The preflush liquid and pre-main acid is not considered. (2) All kinds of liquids are piston drive. (3) All fluids are Newtonian fluids. (4) Acid/rock reaction is not considered.

After injection of viscous acid, the distribution of various fluids is as shown in Fig. 18. The viscosity is μ₁ (viscous acid) in the small circular area near the well and μ₂ (formation fluid) in the circular area far away from the well.

Fig. 18

Radial distribution of fluid in formation after injection of viscous acid

Fig. 19

Longitudinal distribution of fluid in formation after injection of viscous acid

Suppose the pressure on the supply boundary is P_e, the pressure at the bottom of the well is P_w, the pressure at the junction of the two areas is P₁, and the radius is R₁. Therefore, the pressure distribution law in the range of Rw ≤ R ≤ R₁ is as follows:

$$P={P}_1-\frac{Q_1{\mu}_1}{2\pi Kh}\mathit{\ln}\frac{R_1}{r}$$

(2)

The pressure distribution in the annular region where R₁ ≤ r ≤ R_e is:

$$P={P}_e-\frac{Q_2{\mu}_2}{2\pi Kh}\mathit{\ln}\frac{R_e}{r}\mathrm{b}$$

(3)

According to the continuity relationship:

$${Q}_1={Q}_2=Q$$

(4)

The quantity can be calculated:

$$Q=\frac{2\pi Kh\left({P}_e-{P}_w\right)}{\mu_1\mathit{\ln}\frac{R_e}{R_1}+{\mu}_2\mathit{\ln}\frac{R_1}{R_w}}$$

(5)

Q — quantity, P_e — supply edge pressure, P_w — bottom hole wellbore pressure, P₁ — pressure at the interface of viscous acid and formation fluid, μ₁ — viscosity of viscous acid, μ₂ — formation fluid viscosity, R₁ — radius of interface between viscosity acid and formation fluid, R_e — supply edge radius, R_w — wellbore radius, K — reservoir permeability, h — reservoir height.

For the actual formation, as shown in Fig. 19, the formation is longitudinally heterogeneous, assuming that the permeability of the two layers in the figure are K₁ and K₂ respectively, then:

$${Q}_1=\frac{2\pi {K}_1{h}_1\left({P}_e-{P}_w\right)}{\mu_1\mathit{\ln}\frac{R_e}{R_{11}}+{\mu}_2\mathit{\ln}\frac{R_{11}}{R_w}}$$

(6)

$${Q}_2=\frac{2\pi {K}_2{h}_2\left({P}_e-{P}_w\right)}{\mu_1\mathit{\ln}\frac{R_e}{R_{12}}+{\mu}_2\mathit{\ln}\frac{R_{12}}{R_w}}$$

(7)

$$\frac{Q_1}{Q_2}=\frac{K_1}{K_2}\cdot \frac{h_1}{h_2}\cdot \frac{\mu_1\mathit{\;\ln\;}\frac{R_e}{R_{11}}+{\mu}_2\mathit{\;\ln\;}\frac{R_{11}}{R_w}}{\mu_1\mathit{\;\ln\;}\frac{R_e}{R_{12}}+{\mu}_2\mathit{\;\ln\;}\frac{R_{12}}{R_w}}$$

(8)

Q ₁, Q₂ — quantity corresponding to reservoir height h₁ and h₂; K₁, K₂ — permeability of reservoir corresponding to h₁ and h₂; h₁, h₂ — eight of reservoirs with different permeability; R₁₁, R₂₂ — region of junction between viscosity acid and formation fluid in reservoirs with different permeability. Assuming viscosity of viscous temporary plugging acid μ₁ = aμ₂, R₁₂ = bR₁₁ (a > 1), and because K₁ < K₂, and then b > 1:

$${\displaystyle \begin{array}{c}\frac{Q_1}{Q_2}=\frac{K_1}{K_2}\cdot \frac{h_1}{h_2}\cdot \frac{\mu_1\ln \frac{R_e}{R_{11}}+{\mu}_2\ln \frac{R_{11}}{R_w}}{\mu_1\ln \frac{R_e}{R_{12}}+{\mu}_2\ln \frac{R_{12}}{R_w}}=\frac{K_1}{K_2}\cdot \frac{h_1}{h_2}\cdot \frac{a{\mu}_2\ln \frac{R_e}{R_{11}}+{\mu}_2\ln \frac{R_{11}}{R_w}}{a{\mu}_2\ln \frac{R_e}{b{R}_{11}}+{\mu}_2\ln \frac{b{R}_{11}}{R_w}}\\ {}=\frac{K_1}{K_2}\cdot \frac{h_1}{h_2}\cdot \frac{a{\mu}_2\ln \frac{R_e}{R_{11}}+{\mu}_2\ln \frac{R_{11}}{R_w}}{a{\mu}_2\ln \frac{R_e}{R_{11}}+{\mu}_2\ln \frac{R_{11}}{R_w}+\left(1-a\right){\mu}_2\ln b}\end{array}}$$

(9)

Assume, $B=\frac{a{\mu}_2\mathit{\ln}\frac{R_e}{R_{11}}+{\mu}_2\mathit{\ln}\frac{R_{11}}{R_w}}{a{\mu}_2\mathit{\ln}\frac{R_e}{R_{11}}+{\mu}_2\mathit{\ln}\frac{R_{11}}{R_w}+\left(1-a\right){\mu}_2\;ln\;b}$, therefore, the formula of seepage velocity ratio at any point in the formation is as follows:

$$\frac{v_1}{v_2}=\frac{\frac{Q_1}{2\pi {rh}_1}}{\frac{Q_2}{2\pi {rh}_2}}=\frac{K_1}{K_2}\cdot B$$

(10)

Since b > 1, lnb > 0, 1−a < 0, if the viscosity of viscous temporary blocking acid is high enough, that is, a is much higher than 1, then B is larger, and B > 0, then the difference in velocity between the two layers is smaller, to eventually reach the same velocity and uniform distribution acid.