Abstract

Chunhua Block of Shengli Oilfield is one block where shale caving formation， borehole instability， well click， trip sticking， drill pipe blocking and sticking during completion logging and other downhole complex issues were encountered during the drilling process. The paper was based on slow down pressure transmission， filtrate invasion and strong inhibition of cooperation anti-sloughing basic principle. Through optimization and compatibility of the major additives， a new-type drilling fluid is characterized by good rheological properties， filtration loss， lubricity， stability and strong inhibition. The new drilling fluid has been applied in 5 wells for block test. The result showed that all wells gained a satisfactory anti-sloughing effect. The enlargement ratio of well bore diameter was reduced efficiently. The drilling time was saved. And the ratio of successful logging by one time was 100%.

Key words： Drilling fluid； Borehole stability； Shale strata； Cooperation anti-sloughing； Chunhua Block

INTRODUCTION

The formation of Chunhua Block is complex. Wells drilled in this block have encountered several kinds of downhole problems such as borehole collapse and caving， drag and pipe sticking during trip， logging obstructing， sticking， bad cementing quality. There exist easily have hydrating swelling and dispersive red shale layers in Kongdian Formation in Chun-26 Block. During drilling process， the filtrate in drilling fluid soaking will form a soft -shale borehole around the bore hole. Cables and electrical measuring instruments are prone to sticking under the pressure effect in logging process[1～2]. Meanwhile there exist a certain salt-gypsum containing， after entering the segment， salt-gypsum pollution caused by excessive drilling fluid viscosity， bigger dehydration， difficult to control rheology， virtual thick mud cake often formed in the borehole， causing lageniform hole characterized by tight hole and hole washout coexistence， tripping sluggish， logging obstructing， sticking and other complex problems[3～4]. According to incomplete statistics， 90% of the construction wells which use conventional anti-sloughing drilling fluid system， collapsed severely in Shahejie Three， Shahejie Four Formation， the enlargement ratio of bore hole diameter was over 30%， and electrical logging obstructing and sticking in per well waste 96 hours time， which seriously affects the drilling process， consumes lots of manpower and resources， brings huge economic losses. Therefore， developing a high efficient anti-sloughing drilling fluid which adapted to the formation is urgent.

1. LABORATORY ON HIGH EFFICIENT ANTI-SLOUGHING DRILLING FLUID SYSTEM

1.1 Technology Roadmap

Through the analysis of technical data of well construction of Chunhua Oilfield， based on the formation characteristics of the oilfield， the author considers that during drilling with the new drilling fluid system， the following conditions must be met.

（1） During the secondary drilling from Minghua to Dongying formation， due to serious mud problem， continue to use water circulation drilling fluid technology which commonly used in Shengli oilfield， appropriately make a large upper bore hole drilling into the open one.

（2） Drilling to the bottom of Dongying formation， begin to make proper transformation of drilling fluid， reduce the filtration loss， make drilling fluid moving freely.

（3） Because amine polymeric alcohol has a strong inhibitory ability[5～6]， after entering the Shahejie formation， it is added to drilling fluid for improving the inhibition of the drilling fluid， reducing shale hydrating dispersion and erosion.

（4） Because the strong inhibition polysulphonate anti-sloughing drilling fluid system is weak dispersion system[7～8]， particle size distribution of clay particles in system is coarser， need to add some inert rigid particles， fill with clay particles and additives， form dense filter cake， reduce the amount of drilling fluid filtration.

（5） Aluminum flexible polymer and polyalcohol anti-collapse agent are selected to give full play to their chemical solid wall， improve the bearing capacity of the formation， to a certain extent， prevent borehole collapse and lost circulation[9-10].

（6） In order to improve the high temperature capability of the drilling fluid， additives of temperature resistance above 150 ℃ were used in all wells， avoid using pyrolysis additives easily.

1.2 Encapsulation Inhibitor Optimization

In order to select the encapsulation inhibitor which is suitable for the formation， the red shale in Chun-26 block is used to evaluate several additives. The results are shown in Table 1.

From Table 1， we know that for red shale in Chun-26 block， PAM of polymer is the best， followed by the PAC-141， the dispersion effect of polymer encapsulation inhibitor is superior to other classified anti-sloughing agent； lignite products SD-201 places the highest recovery rate； emulsifled modified asphalt is better than FT-1 in asphalt products. In polyalcohol， JHC-1 has the highest recovery rate and the best inhibiting.

1.3 Development of High Efficient Anti-Sloughing Drilling Fluid System

After the system at room temperature and 150 ℃/16h aging properties as shown in Table 2， the experimental results known from the table， good conventional performance， low viscosity， water loss reduction effect is good， high temperature stability. The dispersion recovery rate reached 96.32% for red shale layers of Kongdian Formation of Chun-26 Block.

From Table 7， drilling fluid is optimized with good reservoir protection capacity， the return permeability is above 80% after flow-back-broken down.

2. FIELD APPLICATION

There exists shale， gypsum and salt rock in the deep complex layers. The shale with hydrating， dispersing and swelling characteristics， it is bound to affect the rheology of drilling fluids. Salt gypsum layer dissolved will produce sodium、calcium and magnesium ions， and high concentration of sodium， calcium， magnesium and other ions compressed electric double layer of colloidal particles， so that thin film hydration， resulting in colloidal particles tend to coalesce， destructing stability of the drilling fluid， filtration soared， mud cake thickness， flow variation and even loss. Damage to property of drilling fluid. Salt gypsum layer on the dissolution properties of drilling fluid damage is irreversible， this is bound to increase the performance of drilling fluid and the difficulty of maintenance.

In addition， Shahejie shale formation broken， water sensitivity is weak， bedding is developed and interbeded sandstones and mudstones with different permeability， drilling fluid filtrate easily along the interface into the deep formation， collapse is mainly due to the drilling fluid and filtrate into the internal formation， shale surface hydration， thereby causing brittle shale collapse. While the less compacted shale crushed， high pressure， drilling pressure is released， also easy to cause the borehole collapse.

The new drilling fluid has applied in 5 wells for block test. During the drilling process does not appear any apparent collapse， caving， drilling fluid thickening， sticking and logging obstructing phenomenon. Compared with the adjacent well’s logging curve， the well diameter rules， no “jagged” curve， no “l(fā)ageniform hole”， hole enlargement rate was significantly lower and achieved good results.

（1） Chun-52 well was drilled to TD 2470 m， completion layer is Kongdian with density 1.17 g/cm3. Wells in the field were easy to paste the upper formation， to collapse and caving in the lower Shahejie formation， forming a “complex situation on the paste off”. During drilling in Kongdian formation with long interval， it is prone to sticking and electrical obstructing； especially with a thin layer of gypsum in S4 section and the junction of Kongdian formation， when drilling through it that will make the drilling fluid loss of mobility， API dehydration spurt， causing downhole complex problems.

For sand-gypsum layer in S4 section and Kongdian formation interface， before drilling to the horizon about 50m， adding soda ash 200kg， compound salt filtration reducing agent （LL-JLS） 1.5-2% and organosilicon methyl collapse-preventing viscosity reducing agent （JS-3） 1.5-2.0%， improving the drilling fluid contaminate resistence. From the filtrate analysis shows， Ca2+ content increased 354.21 mg/L， Mg2+ content increased 200.01 mg/L， Cl content was also increased slightly by the analysis of well water and drilling fluid filtrate ion content， it was concluded that drilling gypsum layer， but the drilling fluid flow has been well maintained， apparent viscosity and API dehydration has no change and ensure the construction of the well smoothly.

By using the compound salt resistence drilling fluid system in the application of the well， water expansion，tight hole and electric sticking in red layer Kongdian formation were controlled effectively， the well logging 3 times and the success rate is 100%.

（2） Chun-X103 well was drilled to TD 3291 m， drilling fluid density up to 1.30 g/cm3. This well belongs to a STH of an old well， and the side tracking point is at 2380 m. The water sample analysis showed that Ca2+ contents up to 319.20 mg/L， Mg2+ contents up to 172.63 mg/L， and the content of Cl- is 1650.31 mg/L， so the formation water salinity are on the high side， we still have adopted the compound salt resistence drilling fluid system， and all well construction smoothly， without complex problems.

（3） Chun95-X3 well was drilled to TD 2774 m， and drilling fluid density is 1.23 g/cm3. When the well is drilled to 2000 m， it is gradually changed to compound salt resistence drilling fluid system. At 2180 m， a slight change of fluid loss， by the analysis of drilling fluid filtrate Ca2+ contents up to 378.82 mg/L， Mg2+ contents up to 274.30 mg/L， the Cl- content is 2950.30 mg/L， but drilling fluid performance is stable， complex problems did not occur ， the completion logging is successful one time.

（4） Liang20-30 well was drilled to TD 3230 m and drilling fluid density is 1.15 g/cm3. When drilled to 2050 m tripping， before which API dehydration was 8 mL， going on drilling， API dehydration increased to 20 mL， after adding 1.5 tons of natural polymer drop water loss and 1 tons of polymer fluid loss agent， water loss is still in a high level （API dehydration was 16 mL）， after add 1 ton LL-JLS， dehydration immediately dropped to 5.5 mL. Through the analysis of drilling fluid filtrate， Ca2+ contents up to 499.80 mg/L， Mg2+ contents up to 324.30 mg/L， the Cl- content is 3950.30 mg/L， when drilled in salt water layer， the drilling fluid was converted to compound salt resistence drilling fluid system， well completion operation is smooth.

（5） Liang 20-X31 well was drilled to TD 3308 m， drilling fluid density is 1.15 g/cm3 ； Liang 20-23 well drilled deep 3265 m， density is 1.15 g/cm3. The two wells operation was based on the experience of the Liang 20-30 well， at 1900 m， the drilling fluid can be converted to compound salt resistence drilling fluid system， the result confirmed by drilling fluid filtrate analysis， there exist salt water layer in 2000 m or so， because the preparing work is enough， the drilling fluid performance is operated smoothly.

CONCLUSION AND ADVICE

（1） The new anti-sloughing drilling fluid has good rheological property， small amount of API filtration and lower solid content， its rheological properties can be adjusted by reinforced adhesion agent and flow pattern regulator.

（2） The new anti-sloughing drilling fluid has good temperature resistance， stability at 150 ℃； has strong resistance to salt， calcium， anti soil contamination ability； has good shale inhibition， can effectively prevent the borehole collapse and caving； the permeability recovery value and shale recovery rate is higher， reservoir protection effect is good.

（3） The new anti-sloughing drilling fluid meet the requirement of drilling operation in Chunhua Block. It can effectively prevent accidents and complex problems， and achieve considerable economic benefit.

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