INSIDE THIS ISSUE ➢ Role of Geomechanics in Sand Control ➢ Seeing beyond the borehole with Deep Shear Wave Imaging ➢ E&P Update ➢ Petrophysical and Geomechanical Analysis of wells of a

Satellite field

Quarterly Bulletin

Volume 1 Issue 3

October-December 2020

Role of Geomechanics in Sand Control Sand production is a major concern in many oil and gas fields worldwide. The abrasive nature of sand grains moving within a stream of produced fluids (particularly at high gas rates) means that well completions, production facilities and pipelines may be eroded beyond acceptable and safe limits, even causing discharges and catastrophic failure in equipment. Therefore, sand production can constitute a severe safety risk to oil and gas production operations. Even when damage is limited, the production of solids can cause fill to occur in wells, production lines, separators, top-side facilities, distribution pipelines and process facilities, thereby reducing their output and efficiency. Sand production has been a major topic for geomechanics research, both in the laboratory and in the field, for many years in oil and gas industry. The mechanism is two-stage process. First is the initiation of rock failure caused by stress exerted due to drawdown. This followed by second stage where the failed sand grain is transported to surface in the fluid stream. Sometimes if the fluid velocity is not enough to move the sand grains upwards, it may get deposited at the bottom of the hole. Essentially, three primary mechanisms for sanding have been known for some time; namely fines migration, tensile failure, and shear failure. More recent studies have also revealed a fourth mechanism of compaction or volumetric failure, occurring around wells with highly depleted and porous weak sands, which can lead to sanding. Fines migration reduces permeability in the reservoir formation and in completions (i.e., screens and gravel packs), but the volumes of produced solids involving this mechanism are relatively small. Tensile failure is only significant in already unconsolidated sands, which by and large require sand control from the onset of production, or in otherwise cemented sands which have become disaggregated through the action of shear failure or compaction failure. Once a material is disaggregated, and if the flow gradient at the sand-face is high, tensile forces can cause the unconsolidated material to move into the wellbore. Efficient perforations with low skin damage (e.g. PURE* system) are very effective at mitigating these effects. Shear Failure of otherwise cemented and intact rock may occur whenever the contrast between the minimum and maximum stresses acting in the rock causes its shear strength to be exceeded. Whenever a wellbore is drilled, or when a perforation cavity is created, this stress contrast increases beyond that which exists in the undisturbed rock. The largest magnitude in stress contrast, usually occurring between the effective stresses normal to the cavity face and those either tangential or longitudinal to it, is highly dependent on the orientation of the perforation or well with respect to the in-situ stresses. Therefore, the problem can be mitigated by use of oriented completions and oriented perforations that minimize any stress contrast at the sand face. The stress contrast (and therefore shear failure) is, however, exacerbated further by higher drawdown in the well and by depletion of the reservoir. In fact, this is a common cause of breakout during drilling, where increased mud weight and overbalance can be used to help reduce the stress contrast. Therefore, further mitigation can be achieved during production by reducing the amount of drawdown in the well, but of course this impact on production. Volumetric or compaction failure is also caused by depletion and increased stresses but occurs only in porous and weakly cemented formations where a geometrical collapse of the pore structure occurs. Rather than causing shear failure and vertical and lateral displacements in the rock, the result is a reduction to the pore volume due primarily to vertical compaction. This is the same mechanism that causes the large compactions leading to reservoir collapse and subsidence. High porosity and low mechanical strength increase the likelihood of this volumetric instability. If the failure of the intact rock can be predicted and prevented, then the issue of transport is of no concern. Therefore, a common starting point for most sanding analyses involves in-situ stress analysis, rock strength, pore pressure profile and failure prediction, which are critical components of geomechanics analysis. A wide variety of approaches are available in industry for both single well and field scale evaluation where the sand prediction is undertaken to account for plasticity effects that modify the strength behavior of sands surrounding open-holes and perforations during drawdown and production. These are based on either lab results, theoretical studies or field calibration data. Some of these are analytical, semi-analytical or numerical. The process includes assessing the states of stress (magnitude and direction) at the borehole walls or perforation tunnels, considering orientation and hole sizes, the point of rock failure is then computed based on the mechanical properties (Unconfined compressive strength, Poisson’s Ratio) of the rocks. Stresses acting in the rock and around the borehole walls or perforation tunnels are also adjusted to account for depletion effects. The critical drawdown pressures are obtained for specific completions and specific depletion scenarios. The analyses and results allow comparison of sanding risk for different completion strategies and at different stages in the life of the field, and can identify high-risk zones in the completion interval that should be considered for isolation, that may be particularly susceptible to sanding, or that may be expected to fail further under different production conditions.

SPWLA INDIA CHAPTER

Editorial Column

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SPWLA India

Chapter-Technical

Logging tools have excellent resolution but their depth of investigations is compromised. Deep Shear Wave Imaging (DSWI) is

new technique to visualize geological events away from the borehole. This technique was deployed in well WHRAA in Cachar

region. The well had a consistent poor borehole condition which is attributed to major tectonic activity in this region resulting

in fractures and faults. The lithology is mainly characterized by shale, siltstone and claystone along with sandstone sequences.

With the help of DSWI not only the fractures far off from the bore hole were identified, but geomechanical response of the

formation was also addressed. An acoustic based stress profiling and acoustic anisotropy analysis was carried out to estimate

borehole stability for the drilled section to provide understanding for future drilling program.

Analysis of the shear wave velocities identified the nature of the anisotropy, in terms of either stress induced or caused by

the presence of fractures in the vicinity of the borehole. Earlier this technique was also used in Nohta wells of Vindhyan Basin.

The technique helped to map the fractures away from the borehole to identify the stimulation zones for hydro fracturing.

Selection of suitable stimulation zone considers the role of natural fractures for maximizing the production.

Seeing beyond the vicinity of borehole- A new prospective with Deep Shear Wave

Imaging

Crude Oil Prices

Page 3

SPWLA India

Chapter-E&P News

After the disintegration of Soviet Union Caspian Sea region has become a major source for Hydrocarbon energy

sufficiency to the world. It is categorized with four sedimentary basins. And bordered by Russia, Kazakhstan,

Turkmenistan, Iran and Azerbaijan. The sea is land-locked discharge from seven rivers comes to it with Volga the largest

one. Sea is the economy driver in the region. Hydrocarbon production is over 3 million b/d potential exists for nearly

doubling it in near future if stability in region is stored. There are four sedimentary basins and provinces in the Caspian

region, all of them already proven to be oil and gas productive both onshore and offshore.

Peri-Caspian basin forms a nearly perfect circle of ~500 km diameter, about one quarter of which is under the shallow

waters of Kazakhstan Caspian sector. On the basin periphery there are Paleozoic highs capped by reefs and platform

carbonates, which contain by far largest, most promising yet most challenging fields and prospects in the region2). The

overlain massive section of Permian salt with several hundreds of domes serves as a regional seal and also controls the

structural plan for shallow Mesozoic oil fields. Several world-class oil and gas fields like Tengiz, Karachaganak,

Astrakhan, Zhanazhol were found in the pre-salt Paleozoic section on-shore, each with reserves 1 to 10 bln. bbls of oil

or oil equivalent. The very first offshore well drilled from the drilling barge by OKIOC International Consortium resulted

with discovery of Kashagan field at TD over 4000 m. Second well confirmed the discovery. Good seismic control and

petrophysical analogies with onshore Tengiz and Astrakhan fields indicate reserves of 30 bln. bbls of light, sour crude

with high GOR for this single massive reef. Shallower post-salt Mesozoic leads provide smaller (50- 300 mln. bbls) but

somewhat easier targets with a sweet, heavier crude.

Buzachi trough is between larger Peri-Caspian and North-Caucasus-

Mangystau basins. Most of its heavier oil fields confined to Jurassic stacked

sand reservoirs are located in the transitional zone and therefore

considerable amount of drainage and dike protection is required to maintain

operations. Clearly defined offshore extensions and chains of structures

represent attractive low risk exploration targets.North-Caucasus-Mangystau

basin extends along the northern slopes of Caucasus from Black Sea to

Caspian and further to Kazakhstan. Its main futures include multilayered oil

and gas reservoirs in Mesozoic clastic rocks, high GOR or free gas, light high

quality, yet paraffinous crudes. The recent discovery in the Russian sector

of Khvalynsk field by LUKOil (block Severny) belongs in this tectonic area.

South-Caspian basin also crosses Caspian Sea from shore to shore. It is formed by very thick (up to 20 km) mostly clastic

sedimentary section, usually not older than Mesozoic (the main productive series are associated with Middle Pliocene).

Mud volcanoes and active tectonics are widespread. Basin contains proven oil and gas fields both in Azerbaijan and in

Turkmenistan; some of them discovered over 100 years ago. Its offshore ridge extending from Absheron peninsula in

Azerbaijan to Cheleken in Turkmenistan was mainly drilled during the Soviet times, resulting with the chain of offshore

fields, some of them like Oil-Rocks producing since 1949. Consortium in Azerbaijan (operator BP-Amoco) as well as some

smaller players in Turkmenistan basically took the appraised offshore fields for the development. The large gas-

condensate field Shakh-Deniz recently discovered by BP-Amoco has supported its offshore potential, four sedimentary

basins in the region, each containing substantial proved hydrocarbon reserves. Five independent states (Russia,

Kazakhstan, Azerbaijan, Turkmenistan, and Iran) share these reserves. The primary challenges of the area include: high

cost of oil and gas transportation; territorial disputes; complicated seismic and geological conditions; and sensitive

ecology. Early activity in the region was disappointing; however, several recent oil and gas discoveries, including what

may be the largest offshore oil field in the world (Kashagan in the Kazakhstan sector), may renew interest.

Disclaimer

The material and opinions expressed in this chapter reflect what is believed to be informed opinion, they are not

represented as being the opinions of any regulatory body. Readers are urged to obtain independent advice on

any matter or subject.

E&P Update

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SPWLA India

Chapter-Technical

Nagayalanka field is located in the West Godavari sub-basin on the onland part of the Krishna-Godavari (KG) Basin. This field is being developed jointly by ONGC Ltd (with 51% share) and M/s Cairn India Ltd (with 49% share). ONGC Ltd is the operator for development cum production stage. Drilled three vertical wells viz., X & Y in 1Z block and Z in SE block in phase-I. Gollapalli & Raghavapuram pay sands were developed in wells Y & Z whereas in well X, only Gollapalli pay sand developed Petrophysical Analysis:

From Basic logs, it is very difficult to demarcate OWC with certainty in Gollapalli formation due to fresh water formations where the Resistivity contrast between oil & water is minimal. RT vs DT overlay technique in wells-X, Y, Z (fig-2) and D-T2 plot & T1-T2 spectrum of NMR in well-X (fig-3) failed to identify the contact due to deep OBM invasion in this tight reservoir. 5” Slim Saturn 3D radial probe along with Modular dynamic tester and Insitu-fluid analyzer was lowered by pipe conveyed logging in wells Z and Y (fig-4 & 5) in 5 7/8” slim hole section. Identified OWC from Saturn-IFA. Identification of fluids type by 5” Saturn 3D radial probe: Based on petrophysical analysis, Gollapalli pay thickness of 249m, 194.3m and 77.68m observed in wells X, Y & Z respectively. Observed Raghavapuram pay thickness of 23.41m and 3.35m in Y &Z respectively.

Geo-mechanical analysis: Perforation zones were identified by Geo-mechanical modelling based stress barriers from multi-stage hydrofracturing. After Multi stage HF and flow back wells produced oil Well-X: Qo:136m3/d, Qg:39000m3/d and Qw:3m3/d at FTHP:2600psi through 16/64” choke; Well-Y: Qo:98m3/d, Qg: 14000m3/d and Qw:17m3/d at FTHP:2600 psi through 16/64” choke; Well-Z: Qo:21.2m3/d, Qg: 8891m3/d and Qw:17m3/d at FTHP:600psi through 5mm choke.

Fig-6: Identification of perforation zones in wells X, Y & Z based on stress barriers for multi stage HF

Petrophysical and Geo-mechanical analysis of Wells of a Satellite Field

SPWLA INDIA CHAPTER , 103, 11 High Bui ld ing, Logging Serv ices, ONGC, Bandra -Sion L ink Road, Mumbai – 400017, India Mai l : spwla.india@gmai l.com Ph no: +91-22-24088103