Platelike resin, asphalts (softening point: 140-165^°C), asphalts (softening point: 190-230^°C), fiber JDF(15 mm long) were supplied by the Drilling & Production Research Institute, Petro China Jidong Oilfield Company. Fiber DL-93 (fine) was obtained from SINOPEC Research Institute of Petroleum Engineering, China. Calcium carbonate superfine powder was purchased from Guangzhou Jialiang Minerals Co., Ltd., China. Polymer viscosifier (SDKP) was developed in our laboratory and industrially produced in Chuangxin Science Technology Co., Ltd., Dongying, China.

Dissolution ratio of every lost circulation material was conducted parallel at 80^°C for 4 hours. The sealing characteristics of plugging slurries were evaluated using high temperature high pressure circulation simulation device SDY-2 (Fig. 1). The working pressure of SDY-2 is between 0 and 40 MPa, and working temperature is from ambient temperature to 220^°C. The shear rate of mixing propeller in pollution still is 0-1000 s^-1. The diameter of fracture simulation block is 38 mm, with length of 50 mm. The width of the cracks is from 0.2 mm to 5 mm. Sealing properties of removable plugging slurries were measured using SDY-2 through 1 mm, 2 mm, 3 mm, and 4 mm cracks at room temperature and other setting values.

Fig. (1). Images of parts of the HTHP circulation simulation device SDY-2.

It could be observed from Fig. (2) that fractures were developed on the cylindrical surface of (a) full size cores and inner cylindrical surface of (b) peripheral projection with different distribution angle. The width of fractures ranged from 0.5 mm to 3 mm. The end face of (a) full size cores was penetrated by fractures. Natural fractures and induced fractures were both found in imaging logging (Fig. 3). Distributional patterns of natural fractures included vertical straight seam, netlike cross stitch, oblique fractures with intersection angles. In well NP280, interval of 4722 m-4732 m develops netlike fractures with strong infiltrability, most branches, high angle, good continuity, with average width of 13 μm, length of 3.93 m/m², distribution density of 4.89 strips/m, fracture porosity of 0.98% and accompanied by corrosion holes (Fig. 4).

Fig. (2). Pictures of fractured carbonate cores from oil-bearing formation.

Fig. (3). Fracture features in the imaging logging photographs.

Section of 4542.5 m-4543.5 m has better physical properties and develops most pores. Comprehensive analysis of imaging logging data of Nanpu sag showed that the average fracture width of buried hill carbonate reservoir in 1# structure was 17.5 μm and ranged from 2 μm to 2714.21 μm, fracture width of carbonate in 2# structure ranged from 3.8 μm to 143.56 μm, with average of 40.65 μm, and the average fracture width of carbonate in 5# structure was 3.7 μm (Table 1).

During fluid circulation, near wellbore temperature of buried hill fractured carbonate reservoir is between 50^°C-62^°C in peripheral projection, 125^°C-155^°C in 1# structure, 135^°C-155^°C in 2# structure, 170^°C-200^°C in 3# structure. The temperature tolerance of plugging materials and slurries should meet temperature in formation where they will be used. Combined with reservoir protection, removable plugging slurry was proposed to stop lost circulation. Plugging method for fluid losses in fractured carbonate reservoir is as follows: bridge firstly with fibrous materials, fill the residual gap with granular, platelike, powder and deformable materials to form impermeable, thin sealed belt quickly. The basic components of plugging slurry include fibrous materials of average sizes, larger particles, thinner platelike materials, fine fiber materials and deformable materials.

Results of resolvability test of lost circulation materials were listed in Table 3. Selected platelike resin and asphalts were soluble in crude oil. Their dissolution ratios were all more than 96%. Calcium carbonate superfine powder, fibers JDF (15 mm long), and DL-93 (fine) were all soluble in hydrochloric acid.

To improve suspension property of plugging materials in slurry, polymer SDKP with high temperature tolerance of 220 ^°C was selected as viscosifier. The removable plugging slurry formulas aimed at lost circulation in fractured carbonate reservoirs were designed as follows:

The plugging slurry formula used in 1# and 2# structures: 1 % (w/v) SDKP + 1% (w/v) Fiber JDF + 1.5% (w/v) Platelike resin (6mm) + 2% (w/v) Asphalt (softening point: 140-165^°C, 3mm) + 1% (w/v) Platelike resin (1mm) + 4% (w/v) Fiber DL-93 + 4% (w/v) Calcium carbonate superfine powder (denoted by KJD155).

The plugging slurry formula used in 3# structure: 1 % (w/v) SDKP + 2% (w/v) Fiber JDF + 2% (w/v) Asphalt (softening point: 190-230 ^°C, 5mm) + 2% (w/v) Asphalt (softening point: 190-230 ^°C, 3mm) + 1% (w/v) Asphalt (softening point: 190-230 ^°C, 1mm) + 4% (w/v) Fiber DL-93 + 4% (w/v) Calcium carbonate superfine powder (denoted by KJD200)

The total dissolution rates (acid soluble and oil soluble) of the formula KJD155 and KJD200 are 83.18 % and 78.71%. Therefore, sealing belt composed of these lost circulation materials can be removed by acid pickling and dissolving in crude oil.

Results of sealing ability evaluation of removable plugging slurry KJD155 are presented in Table 4. For 1 mm crack block, cumulative leakage of slurry KJD155 was zero at room temperature and 120 ^°C, 6.8 mL at 150 ^°C when pressure was up to 5 MPa. For 2 mm crack block, cumulative leakage at room temperature, 120 ^°C, 150 ^°C was 1.8 mL, 2.5 mL, 9.9 mL separately. For 3 mm crack block, cumulative leaked volume at room temperature, 120 ^°C, 150^°C was 0.7 mL, 1 mL, 7.3 mL respectively. For 4 mm fracture module, cumulative leakage at room temperature, 120 ^°C, 150^°C was 0.5 mL, 0.8 mL, and 6.3 mL separately. The formula KJD155 sealed 1 mm, 2 mm, 3 mm, and 4 mm crack blocks at room temperature, 120 ^°C and 150 ^°C. It could be seen from Table 5 that the formula KJD200 plugged 1 mm, 2 mm, 3 mm, 4 mm crack blocks at room temperature, 180 ^°C and 200 ^°C effectively. At room temperature, the cumulative leakage of formula KJD200 through 1 mm, 2 mm, 3 mm, and 4 mm crack blocks was 0, 4.8 mL, 1.6 mL, 5.8 mL separately. At 180 ^°C, the cumulative leakage of formula KJD200 through 1 mm, 2 mm, 3 mm, and 4 mm crack blocks was 0, 2.8 mL, 2.6 mL, 3.0 mL respectively. At 200 ^°C, the cumulative leakage of formula KJD200 through 1 mm, 2 mm, 3 mm, and 4 mm crack blocks was 4.4, 9.8 mL, 8.4 mL, and 7.8 mL separately. Before pressuring, cumulative leakage of formulas KJD155 and KJD200 was 0 or small value through different leakage medium, showing that they bridged and filled quickly in fractures to prevent fluid leaking. For the same crack block, cumulative leakages of formulas KJD155 and KJD200 at higher temperature were greater than that at lower temperature, but they were smaller than 10.0 mL (Fig. 7). Bearing pressure ability of plugging slurries KJD155 and KJD200 was up to 5 MPa, which was beneficial to the next operation such as continuative drilling or completion.

Fig. (7). Relation between Ff and filtration rate (diameter of particle is 100 µm).

Well NP23-P2006, a horizontal producing well for developing Ordovician fractured carbonate reservoir, is located at Nanpu sag. Severe lost circulation was encountered during drilling through buried hill at the depth of 5162.98 m. There was no mud return while circulation. The removable plugging slurry KJD200 and cement were adopted after conventional leak stopping was attempted 6 times and failed. Volume of 50 m³ of removable plugging slurries was prepared out of 40 m³ which were pumped into wellbore, followed by 20 m³ cement. The pump pressure did not cut down when stand pipe pressure test was conducted up to 20 MPa, indicating successful sealing operation. Another total loss occured when the well was drilled to the depth of 5179 m. 50 m³ removable plugging slurries were pumped into borehole. The lost circulation was stopped and the circulation of drilling fluid was recovered. The high production capacity was gained during formation testing, showing that the plugging slurries in fractures were removed partly by crude oil and caused small damage to reservoir.

Well G3102-9, located at the land area of Bohai Bay, is a directional well for production in Shahejie 3 formation, with design depth of 3937 m. There were no drilling fluids returned when the well was drilled to the depth of 3256 m. Operation of pulling out was performed at once and the drilling fluid was pumped in continuously. However, liquid level in borehole descended once pump was stopped. Loss rate was up to 53.4 m³/h. Volume of 70 m³ removable plugging slurries (KJD155) were pumped into downhole. Lost circulation rate decreased to 18 m³/h after static sealing for 18 hours. Another 70 m³ removable plugging slurries (KJD155) with extra 7 % (w/v) expansible plugging materials were prepared and pumped into wellbore. Drilling fluid circulation was built after static sealing for 13 hours, showing that the fluid loss was prevented successfully.