Prediction Analysis of Induced Casing Deformation of Horizontal Well in Fractured Shale Reservoir: Case Study of Weiyuan Gasfield

Casing deformation (CD) is a major challenge for shale gas development in Weiyuan gasfield, natural fracture (NF) slippage is one of the main causes of CD in Weiyuan gas filed. In order to study the mechanism and regularity of NF slippage induced CD, a wellbore shear stress calculation model and a CD degree prediction model are established. And results show that, the approach angle and ground principal stress difference have significant influence on wellbore shear stress, high wellbore shear stress occurs when wellbore orientation is perpendicular to the NF trend. Wellbore shear stress increases with the increase of fracture fluid pressure and NF area, improving casing strength or cementing quality has limited effect on reducing the risk of CD. The smaller the young's modulus, the higher the CD degree, Poisson's ratio has limited effect on CD degree. NF approach and fracture fluid pressure determines the value of CD degree. Field case shows that reasonable fracturing technology to control fracture net pressure and wellbore position arrangement are helpful for reducing CD risk, and the model proposed in this paper can be used to predict CD risk and calculate the CD degree.

Influence of Volume Fracturing on Casing Stress in Horizontal Wells

In horizontal wells, the casing string is affected by the gravity effect, temperature effect, swelling effect, bending effect, friction effect and other mechanical effects. In view of this situation, the mathematical models of casing swelling effect and temperature effect caused by volume fracturing are established. The case analysis shows that the length of the unsealed section in the vertical section has a great influence on the axial shortening of the casing during fracturing. With the increase of the unsealed section length, the axial shortening of the casing increases gradually under the same wellhead pressure. In the process of fracturing, repeated squeezing and pressurization lead to periodic changes of the wellhead pressure, casing deformation and load, which leads to fatigue damage and even fracture of casing. At the same time, a large amount of fracturing fluid is continuously injected through the casing during the fracturing process, which makes the wellbore temperature change greatly. The additional stress caused by the temperature change reduces the casing strength, which has an important impact on the wellbore integrity. The mathematical model of temperature stress and its effect on the casing strength during volume fracturing is established. With the increase of the temperature stress acting on the casing, the casing collapse strength decreases gradually. When the temperature stress reaches 200 MPa, the casing collapse strength decreases to 84% of the original. The research results can provide a reference for the casing integrity design and control in the horizontal well fracturing process.

CAUSES OF POOR QUALITY CASING OF DIRECTIONAL WELLS AT THE DRILLING DEPARTMENT «UKRBURGAZ» AND METODS FOR THEIR ELIMINATION

The analysis of modern studies and field data relating to directional wells casing at the enterprises of “Ukrburgas” drilling company has been carried out in the article. In particular, complications with casing strings, which happened when casing directional wells at these enterprises, data on the non-admission of casing strings to the total depth, a brief description of works aimed at restoring the quality of well casing, are considered. Main reasons of complications, damages and failures during well casing process have been determined. It has been found out that casing string failures mostly happened in the areas of intensive wellbore deviation. The reasons of directional wells poor quality casing taking into account the results of new theoretical, experimental and field research have been systemized. Cases of wells poor quality casing are supposed to be divided into two groups: 1) casing strings damage, 2) non-admission of casing strings to the total depth and violation of string integrity. Each of the groups is classified in accordance with the types of poor casing. The main reason of directional wells poor quality casing are failures caused by wrong casing strength calculations, violation of casing well technology, casing strings wear, mining and geological conditions. The non-admission of the columns to the total depth and poor-quality joining of sections are minor, which arise due to the low passability of the well, inaccurate determination of the depth of the well, geological conditions, etc. Measures to prevent complications, damages and accidents during well casing, in particular, work on forming the necessary wellbore profile, maintaining the design parameters of drilling fluid, and compliance with the requirements of enterprise standards during preparation works, lowering, cementing and final casing works have been worked out.

Numerical simulation of the double pits stress concentration in a curved casing inner surface

Sour or sweet oil fields development is common in recent years. Casing and tubing are usually subjected to pitting corrosion because of exposure to the strong corrosion species, such as CO2, H2S, and saline water. When the corrosion pits formed in the casing inner surface, localized stress concentration will occur and the casing strength will be degraded. Thus, it is essential to evaluate the degree of stress concentration factor accurately. This article performed a numerical simulation on double pits stress concentration factor in a curved inner surface using the finite element software ABAQUS. The results show that the stress concentration factor of double pits mainly depends on the ratio of two pits distance to the pit radius ( L/R). It should not be only assessed by the absolute distance between the two pits. When the two pits are close and tangent, the maximum stress concentration factor will appear on the inner tangential edges. Stress concentration increased by double pits in a curved casing inner surface is more serious than that in a flat surface. A correction factor of 1.9 was recommended in the curved inner surface double pits stress concentration factor predict model.