Characterization and prevention of formation damage for fractured carbonate reservoir formations with low permeability

Stress sensitivity and water blocking in fractured carbonate reservoir formations with low permeability were determined as the main potential damage mechanisms during drilling and completion operations in the ancient buried hill Ordovician reservoirs in the Tarim Basin. Geological structure, lithology, porosity, permeability and mineral components all affect the potential for formation damage. The experimental results showed that the permeability loss was 83.8%-98.6% caused by stress sensitivity, and was 27.9%-48.1% caused by water blocking. Based on the experimental results, several main conclusions concerning stress sensitivity can be drawn as follows： the lower the core permeability and the smaller the core fracture width, the higher the stress sensitivity. Also, stress sensitivity results in lag effect for both permeability recovery and fracture closure. Aimed at the mechanisms of formation damage, a modified low-damage mixed metal hydroxide （MMH） drilling fluid system was developed, which was mainly composed of low-fluorescence shale control agent, filtration control agent, lowfluorescence lubricant and surfactant. The results of experimental evaluation and field test showed that the newly-developed drilling fluid and engineering techniques provided could dramatically increase the return permeability （over 85%） of core samples. This drilling fluid had such advantages as good rheological and lubricating properties, high temperature stability, and low filtration rate （API filtration less than 5 ml after aging at 120 ℃ for 4 hours）. Therefore, fractured carbonate formations with low permeability could be protected effectively when drilling with the newly-developed drilling fluid. Meanwhile, field test showed that both penetration rate and bore stability were improved and the soaking time of the drilling fluid with formation was sharply shortened, indicating that the modified MMH drilling fluid could meet the requirements of drilling engineering and geology.

Plugging performance and mechanism of an oil-absorbing gel for lost circulation control while drilling in fractured formations

Lost circulation of drilling fluid is one of the most common engineering problems in the drilling process of fractured formations.In this study,an oil-absorbing polymer gel synthesized using compound monomers with rigid and flexible chains was applied to control the oil-based drilling fluid loss while drilling.The microstructure,oil-absorbing performance,and plugging performance the gel was investigated.A large number of dense pores on the surface of the gel were observed,which allowed the oil molecules to enter the internal space of the gel.The initial oil absorption capacity of the gel was fast,and it increased with the increase in the temperature and decrease in the particle size,reaching 20.93 g/g at140℃.At a high temperature of 140℃,the bearing pressure capacity of the gel formula containing particles of different particle sizes reached 7.6 MPa for a fracture of a width of 3 mm,showing that the oil-absorbing gel have excellent plugging performance at high temperature.Plugging mechanism of the gel was investigated through visualized fracture plugging experiments.Results show that the dynamic migratio n,particle-swelling,particle-bridging,particle-aggregation,deformation-filling,and compaction-plugging contribute to the whole lost circulation control process,reflecting that the plugging performance can be effectively enhanced by improving the aggregation and filling degrees of the gel with different particle sizes.

Formation of inter-salt overpressure fractures and their significances to shale oil and gas: A case study of the third member of Paleogene Shahejie Formation in Dongpu sag, Bohai Bay Basin

Taking the inter-salt organic-rich shales in the third member of Paleogene Shahejie Formation(Es3)of Dongpu sag in Bohai Bay Basin as an example,the origin of overpressure,development characteristics,formation and evolution mechanism,formation stages and geological significance on shale oil and gas of overpressure fractures in the inter-salt shale reservoir were investigated by means of thin section identification,scanning electron microscopy observation,analysis of fluid inclusions,logging data analysis,and formation pressure inversion.The results show that overpressure is universal in the salt-lake basin of Dongpu sag,and under-compaction caused by the sealing of salt-gypsum layer,pressurization due to hydrocarbon generation,transformation and dehydration of clay minerals,and fault sealing are the 4 main factors leading to the occurrence of overpressure.The overpressure fractures are small in scale,with an average length of 356.2μm and an average underground opening of 11.6μm.But they are densely developed,with an average surface density of 0.76 cm/cm2.Moreover,they are often accompanied by oil and gas charging,and thus high in effectiveness.Overpressure fractures were mainly formed during two periods of large-scale oil and gas charging,approximately 25-30 Ma ago and 0-5 Ma ago.Inter-salt overpressure fractures play dual roles as the storage space and migration paths of shale oil and gas.They contribute 22.3%to the porosity of shale reservoir and 51.4%to the permeability.They can connect fracture systems of multiple scales,greatly improving the quality of shale reservoir.During the development of shale oil and gas,inter-salt overpressure fractures can affect the extension and morphology of hydraulic fractures,giving rise to complex and highly permeable volumetric fracture networks,improving hydraulic fracturing effect and enhancing shale oil and gas productivity.

Formation mechanisms of fracture plugging zone and optimization of plugging particles

As formation mechanisms of plugging zone and criteria for fracture plugging remain unclear,plugging experiments and methods testing granular material mechanical properties are used to study the formation process of the plugging zone in fractured formations,analyze composition and ratios of different sizes of particles in the plugging zone,and reveal the essence and driving energy of the formation and damage of the plugging zone.New criteria for selecting lost circulation materials are proposed.The research results show that the formation of the plugging zone has undergone a process from inertial flow,elastic flow,to quasi-static flow.The plugging zone is composed of fracture mouth plugging particles,bridging particles and filling particles,and the proportion of the three types of particles is an important basis for designing drilling fluid loss control formula.The essence of the construction of the plugging zone is non-equilibrium Jamming phase transition.The response of the plugging zone particle system to pressure is driven by entropy force;the greater the entropy,the more stable the plugging zone.Lost circulation control formula optimized according to the new criteria has better plugging effect than the formula made according to conventional plugging rules and effectively improves the pressure-bearing capacity of the plugging zone.The research results provide a theoretical and technical basis for the lost circulation control of fractured formations.

Shale oil enrichment evaluation and production law in Gulong Sag,Songliao Basin,NE China

Based on the results of drilling,tests and simulation experiments,the shales of the Cretaceous Qingshankou Formation in the Gulong Sag of the Songliao Basin are discussed with respect to hydrocarbon generation evolution,shale oil occurrence,and pore/fracture evolution mechanism.In conjunction with a substantial amount of oil testing and production data,the Gulong shale oil enrichment layers are evaluated and the production behaviors and decline law are analyzed.The results are drawn in four aspects.First,the Gulong shales are in the stage of extensive hydrocarbon expulsion when R_(0) is 1.0%-1.2%,with the peak hydrocarbon expulsion efficiency of 49.5%approximately.In the low-medium maturity stage,shale oil migrates from kerogen to rocks and organic pores/fractures.In the medium-high maturity stage,shale oil transforms from adsorbed state to free state.Second,the clay mineral intergranular pores/fractures,dissolution pores,and organic pores make up the majority of the pore structure.During the transformation,clay minerals undergo significant intergranular pore/fracture development between the minerals such as illite and illite/smectite mixed layer.A network of pores/fractures is formed by organic matter cracking.Third,free hydrocarbon content,effective porosity,total porosity,and brittle mineral content are the core indicators for the evaluation of shale oil enrichment layers.Class-I layers are defined as free hydrocarbon content equal or greater than 6.0 mg/g,effective porosity equal or greater than 3.5%,total porosity equal or greater than 8.0%,and brittle mineral content equal or greater than 50%.It is believed that the favourable oil layers are Q2-Q3 and Q8-Q9.Fourth,the horizontal wells in the core area of the light oil zone exhibit a high cumulative production in the first year,and present a hyperbolic production decline pattern,with the decline index of 0.85-0.95,the first-year decline rate of 14.5%-26.5%,and the single-well estimated ultimate recovery(EUR)greater than 2.0×10^(4)t.In practical exploration and production,more efforts will be devoted to the clarification of hydrocarbon generation and expulsion mechanisms,accurate testing of porosity and hydrocarbon content/phase of shale under formation conditions,precise delineation of the boundary of enrichment area,relationship between mechanical properties and stimulated reservoir volume,and enhanced oil recovery,in order to improve the EUR and achieve a large-scale,efficient development of shale oil.

Enhanced oil recovery by low-salinity water spontaneous imbibition (LSW-SI) in a typical tight sandstone formation of mahu sag from core scale to field scale

Accelerating mass exchange between matrix and fractures is the essence of enhanced oil recovery(EOR)in tight formations after natural depletion.Low salinity water(LSW)injection has been commerciallyproven in conventional reservoirs EOR,with scale projects in progress worldwide.There is,however,a lack of understanding of the EOR effect in tight formations.Therefore,in this work,we introduced LSWEOR to a target tight formation using huff-puff mode.Spontaneous imbibition(SI)tests were firstly performed on homogenous Berea sandstone cores with decreasing salinity brine to observe the production response.The results indicated that the oil recovery of the tight rock was boosted by tuning brine salinity.Of all the used brines with salinity ranging from 0.021%to 2.1%TDS(total dissolved salinity),the 0.21%TDS brine showed a rapid increase in oil production over imbibing time,which finally led to an incremental oil recovery of 4.5%OOIP(original oil in place).Core-scale modeling was conducted by history-matching the oil recovery dynamics of the SI results through modifying capillary pressure and relative permeability.A full-scale reservoir model was constructed using micro-seismic data to model fracture geometry combing fracturing results and scaling parameters obtained from core scale historymatching.It is proven that LSW huff-n-puff stimulated the oil production after natural depletion and improved MEE(mass exchange efficiency)of the target formation,but the EOR benefit was not comparable to CO2 and surfactant-assisted water huff-puff methods.

Performance evaluation of nanosilica derived from agro-waste as lost circulation agent in water-based mud

Seepage or loss of the mix-water from the drilling muds into the porous and permeable formations is a common problem during drilling operation.The drilling mud design requires a good knowledge of sealing integrity and all the factors influencing the mud to bridge through fractures or pore throat of exposed rocks.Loss circulation materials(LCMs)are commonly introduced into the drilling mud to prevent or minimize filtrate loss.This study investigates silica nanoparticle(SNP)derived from rice husk(RH)termed RH-SNP using the wet-milling method as an LCM inwater-based mud(WBM).The impact of the RH-SNP in the enhancement of rheology and filtrate loss control properties of WBM was studied.Subsequently,the sealing integrity of the RH-SNP in a 1 mm and 2 mm simulated fracture for 7 min was determined using a stainless-steel slotted filter disk.The performance of the developed RH-SNP was compared with the widely applied nutshell.The synthesized RH-SNP at amount of 2.0 wt% significantly enhanced the yield point and plastic viscosity of the WBM by 75% and 386%,respectively,and minimized the fluid loss of the WBM by 47% at 80°F.The enhancement is due to the particles ability to spread and interact efficiently with the WBM.With the use of 1 mm and 2 mm simulated fracture for 7 min,the mud loss volume of the base mud reduced by 50%,66.7%,86%,and 90%(for 1 mm)and 40%,65.7%,77.1%,and 80%(for 2 mm)with the inclusion of 0.5 wt%,1.0 wt%,1.5 wt%,and 2.0 wt% of RH-SNP,respectively.Overall,the results showed that RH-SNP enhanced the seal integrity of the drilling mud and was more resistant to deformation compared to the nutshell.The findings of this study can help for better understanding of the application of RH-SNP as a loss circulation agent owing to its superior ability to seal fractured formation compared with the often used nutshell.