Based on fractures stress sensitivity, this paper experimentally studies fracture-supported shielding temporary plugging drill-in fluid (FSDIF) in order to protect fractured and fracture-pore type formation. Experimental results show the FSDIF was better than the CDIF for protecting fractured and fracture-pore type reservoir and the FSDIF temporary plugging rate was above 99%, temporary plugging ring strength was greater than 15 MPa, and return permeability was 91.35% and 120.83% before and after acidizing, respectively. The reasons for the better reservoir protection effect were analyzed. Theoretical and experiment studies conducted indicated that the FSDIF contained acid-soluble and non-acid-soluble temporary shielding agents; non-acid-soluble temporary shielding agents had high hardness and temporary plugging particles size was matched to the formation fracture width and pore throat size.
A fractured reservoir is a reservoir in which the fractures act as the main reservoir space and seepage channel. A fracture-pore type reservoir is a reservoir in which the pore acts as the main reservoir space and fractures are the main seepage channel [
The FSDIF formulation was as follows: base mud + 3% filtrate loss reducer + 3% lubricant agent + 6% weighting agent + 0.5% viscosity reducer + 3% formation protective agent + 2% proppant agent.
The CDIF formulation was as follows: base mud + 3% filtrate loss reducer + 3% lubricant agent + 6% weighting agent + 0.5% viscosity reducer + 3% formation protective agent.
Carbonate rock samples from the Changxing group (2 pieces) and Feixianguan group (1 piece) with different permeability levels were used. Fracture samples were confined under a pressure of 30 MPa for 20 minutes. The physical parameters of the rocks are shown in Table
Physical properties of the rock samples.
Rock samples | Stratum | Length (cm) | Diameter (cm) | Porosity (%) | Water permeability (10−3 |
Remarks |
---|---|---|---|---|---|---|
1# | P2ch | 4.540 | 2.53 | 5.05 | 731.89 | Fracture |
2# | P2ch | 5.742 | 2.51 | 10.30 | 27.45 | Fracture |
3# | T1f | 4.620 | 2.52 | 13.61 | 34.22 | Block |
Experimental FSDIF properties.
|
pH | FL (ml) |
|
|
AV (mPa⋅s) | PV (mPa⋅s) | YP (MPa⋅s) |
---|---|---|---|---|---|---|---|
1.58 | 10 | 1.8 | 0.3 | 2/5.5 | 57 | 47 | 10.3 |
The instruments used to study the FSDIF properties included a six speed rotary viscometer, middle pressure tester, density meter, and an automatic measuring instrument for filter cake thickness and toughness. The test methods are detailed in the People’s Republic of China Petroleum and Natural Gas Industries Standards (GB/T16783.1-2014).
The acidification filter cake clean-up method was performed as follows: (1) The temporary plugging rate was determined, and the temporary plugging effect was analyzed using the experiment method of temporary plugging and natural flowback detailed in literature [
The same shielding temporary plugging drill-in fluid was used as in Section
The fracture rock samples used were artificial fracture samples. Before the experiment, fracture rock was confined under a pressure of 30 MPa for 20 min. The physical properties of some samples are shown in Table
Physical properties of rock samples.
Rock samples | Stratum | Length (cm) | Diameter (cm) | Porosity (%) | Water permeability (10−3 |
Remarks |
---|---|---|---|---|---|---|
4# | T1f | 3.592 | 2.520 | 10.470 | 349.000 | Block |
5# | T1f | 3.576 | 2.530 | 9.910 | 311.000 | Block |
6# | T1f | 5.334 | 2.560 | 9.300 | 0.214 | Fracture |
7# | T1f | 7.278 | 2.558 | 11.037 | 0.073 | Fracture |
Experimental results are shown in Table
Evaluation results of the temporary plugging performance of the shielding ring.
Rock samples | Stratum | Fractures width ( |
|
|
Expected temporary plugging rate (%) | Remarks | |||
---|---|---|---|---|---|---|---|---|---|
7.5 | 15 | 30 | 60 | 15 | |||||
1# | P2ch | 61.1 | 0.18 | 0.18 | 0.21 | 0.22 | 100 | 99.94 | Fracture |
2# | P2ch | 20.3 | 0.15 | 0.44 | 0.58 | 0.58 | 99.86 | 98.39 | Fracture |
3# | T1f | — | 0.07 | 0.27 | 0.31 | 0.35 | 99.71 | 98.72 | Block |
The relationship between drill-in fluid loss and time after the formation of shielding ring.
Acidification was an effective measure to remove the filter cake damage in the carbonate reservoir. Therefore, in order to improve the return permeability of the rock samples, the filter cake was removed by hydrochloric acid and the return permeability of the CDIF was compared with that of the FSDIF. As shown in Table
CDIF-FSDIF flowback performance of the shielding ring.
Working fluid class | Rock samples | Stratum | Fractures width ( |
Zd (%) | Before acidification |
After acidification |
Remarks | ||||
---|---|---|---|---|---|---|---|---|---|---|---|
0.5 | 1 | 1.5 | 0.1 | 0.5 | 1 | ||||||
FSDIF | 1# | P2ch | 61.1 | 100.00 | 87.72 | 89.34 | — | 98.35 | 110.28 | — | Fracture |
2# | P2ch | 20.3 | 99.86 | 88.07 | 90.56 | — | 88.72 | 120.83 | — | Fracture | |
3# | T1f | — | 99.71 | 71.23 | 91.35 | — | 73.86 | 105.37 | — | Block | |
|
|||||||||||
CDIF | 8# | T1f | 49.6 | 99.99 | 71.23 | 75.41 | — | 67.35 | 80.26 | 98.32 | Fracture |
9# | P2ch | 41.8 | 99.96 | 59.25 | 83.14 | 90.73 | 61.28 | 95.61 | 93.45 | Fracture | |
10# | T1f | — | 99.83 | 69.57 | 94.28 | — | 61.43 | 104.52 | — | Block |
Performance comparison of the CDIF and FSDIF.
The FSDIF showed obvious advantages over the CDIF such as a significant increase in return permeability of fracture rock samples after acidification.
In forming the carbonate rock, the same open hole section often had both high and low-pressure layers. Loss of circulation was encountered in the low-pressure layers when the high pressure layers were drilling. Therefore, the bearing capacity of this kind of easy leakage formation must be considered [
The RTR-100 triaxial rock mechanics-testing system was used to measure the strength of the shield ring formed using the FSDIF and it was compared with the temporary plugging strength of the CDIF. Experimental results are shown in Table
Table contrasting the shielding ring strength of the CDIF and FSDIF.
Working fluid class | Rock samples | Stratum | Zd (%) |
|
|
|
Click through time statistics | Remarks |
---|---|---|---|---|---|---|---|---|
FSDIF | 5# | T1f | 100.00 | 17.5 | 35 | 130 | 30 min/20 MPa | Block |
30 min/25 MPa | ||||||||
30 min/30 MPa | ||||||||
40 min/35 MPa | ||||||||
7# | T1f | 99.86 | 17.5 | 30 | 83 | 30 min/20 MPa | Fracture | |
30 min/25 MPa | ||||||||
23 min/30 MPa | ||||||||
|
||||||||
CDIF | 6# | T1f | 99.99 | 17.5 | 25 | 47 | 30 min/20 MPa | Fracture |
17 min/25 MPa | ||||||||
4# | T1f | 99.96 | 17.5 | 30 | 86 | 30 min/20 MPa | Block | |
30 min/25 MPa | ||||||||
26 min/30 MPa |
Figure
Grain size distribution curve of the FSDIF.
Table
Pore throat characteristics before and after dissolution of the temporary plugging agent in acid.
Before acidification
After acidification
Fracture characteristics before and after dissolution of the temporary plugging agent in acid.
Before acidification
After acidification
Compared to the CDIF, the FSDIF could significantly improve the return permeability of fracture rock samples after acidification. Since the FSDIF added a certain amount of non-acid-soluble ceramsite, the grain size and fracture width matched. As shown in Figure
Fracture profile characteristics before and after the addition of an integrated temporary plugging agent.
Before acidification
After acidification
Figure
Strength of the filter cake formed by materials with different hardness (strengths) (according to Aadnoy et al., 2007).
The FSDIF could effectively block off fracture and it also has higher strength. The FSDIF added the ceramsite whose strength was higher than that of ultrafine CaCO3 added by the CDIF and at the same time the sphericity of ceramsite increased to more than 99%, which is much higher than the sphericity of superfine CaCO3. With high sphericity and strength of the FSDIF temporary plugging particles, the formation of the shield ring strength will also be higher [
(1) The strong stress sensitivity of fracture was the main reason for the low return permeability of fractured and fracture-pore type reservoirs. Pure acid-soluble temporary plugging agents for the protection of fractured and fracture-pore type reservoirs were not ideal.
(2) Integrated acid-soluble and non-acid-soluble temporary plugging agents of FSDIF exhibited a good temporary plugging effect and return permeability. The temporary plugging rate was 99.5%, the filter cake could bear at least the positive differential pressure of 15 MPa, and the return permeability of the rock samples was 90.42% on average, while the fracture rock return permeability was lower than the base rock samples.
(3) The non-acid-soluble temporary plugging agent was able to support fractures during filter cake removal, to prevent fractures closing, and to reduce the stress sensitivity of the fractured and fracture-pore type reservoirs. This was the main reason for the better protection of fractured and fracture-pore type reservoir by FSDIF with integrated acid-soluble and non-acid-soluble temporary plugging agents.
Density (g/cm3)
Degree of acidity and alkalinity
Loss of water (ml)
Filter cake thickness (mm)
Initial shear stress (Pa)
Ultimate shear (Pa)
Apparent viscosity (mPa·s)
Plastic viscosity (mPa·s)
Yield point (Pa)
Flowback pressure difference (MPa)
Return permeability (%)
Temporary plugging rate (%)
Differential pressure (MPa)
Pressure (MPa)
Time (min)
Water permeability (mD).
The authors declare that there are no competing interests regarding the publication of this paper.
The authors wish to acknowledge that this work was supported by the Natural Science Foundation of the Guangdong Province “study on the mechanism of multiscale mass transfer in coalbed methane damage with fluid damage in the well” (no. 2016A030307024), the Opening Foundation of the Guangdong Provincial in the China Key Laboratory of Petrochemical Equipment Fault Diagnosis “corrosion mechanism and corrosion inhibitor of carbon dioxide in oil and gas wells” (no. GDUPTKLAB201603), the Opening Foundation of Maoming in the China Petrochemical Corrosion and Safety Engineering Technology Research and Development Center “corrosion behavior of oil and gas well pipe under HTHP and concentration of CO2 medium” (no. 660011), College Students Innovation and Entrepreneurship Project “design and degradation performance study of CBM-recycled microbubbles drill-in fluid” (no. 201511656008), and the Chongqing foundation and advanced research project (no. cstc2015jcyjA90022).