Sains Malaysiana 44(10)(2015): 1377–1388

A Finite Element Model to Predict Wellbore Fracture Pressure with Acid Damage

(Model Unsur Terhingga untuk Meramalkan Retak Tekanan Telaga Gerudi dengan Kerosakan Asid)

FANHUI-ZENG*, JIANCHUN-GUO & YUXUAN-LIU

State Key Laboratory of Oil and Gas Geology and Exploration, Southwest Petroleum University, 610500 Chengdu, P.R. China

Received: 13 June 2013/Accepted: 4 August 2015

ABSTRACT

Hydraulic fracturing becomes more difficult when confronted with a formation of high fracturing pressure. In such formations, acidizing before the main fracturing treatment provide a method to reduce fracture pressure. The aim of this paper was to investigate the evolution of fracture pressure in a wellbore with acidizing. Five experiments were conducted to study the mechanisms of acid damage on reservoir minerals and cementing materials properties. Consequently, a mathematical model to predict fracture pressure with acidizing has been established and verified by field data. The analysis results showed that it is possible to reduce fracture pressure with decreased rock strength and fracture critical stress intensity factor by means of acid damage. Acid damage destroys the crystal structure of mineral particles, breaks the crystalline layers in cementing materials, increases rock porosity and reduces the rock strength. In addition, as the acid concentration, formation temperature and acid treatment time increased, it was useful to reduce fracture pressure in the wellbore. Using the proposed model, we were able to select the optimal acid damage construction parameters to reduce fracture pressure.

Keywords: Acid damage; fracture pressure; hydraulic fracture; mechanisms of acid damage; prediction model

ABSTRAK

Keretakan hidraulik menjadi sukar apabila berhadapan dengan pembentukan tekanan keretakan tinggi. Dalam pembentukan itu, pengasidan sebelum rawatan keretakan utama merupakan suatu kaedah bagi mengurangkan tekanan retak. Tujuan kajian ini adalah untuk mengkaji evolusi tekanan retak dalam telaga gerudi dengan pengasidan. Lima kajian telah dijalankan untuk mengkaji mekanisme kerosakan asid dalam tangki mineral dan sifat bahan penyimenan. Oleh yang demikian, model matematik untuk meramalkan tekanan retak dengan pengasidan telah dibangun dan dibuktikan melalui data lapangan. Keputusan analisis menunjukkan bahawa tekanan retak dapat dikurangkan dengan mengurangkan kekuatan batu dan faktor keamatan tekanan kritikal retak melalui kerosakan asid. Kerosakan asid memusnahkan struktur kristal zarah mineral, memecahkan lapisan kristal dalam bahan penyimenan, meningkatkan keliangan batu dan mengurangkan kekuatan batu. Sebagai tambahan, semasa kepekatan asid, suhu pembentukan dan tempoh rawatan asid meningkat, adalah disarankan tekanan retak dikurangkan dalam telaga gerudi. Berdasarkan model yang dicadangkan, kami dapat memilih parameter penghasilan kerosakan asid yang optimum untuk mengurangkan tekanan retak.

Kata kunci: Kerosakan asid; mekanisme kerosakan asid; model ramalan; retak hidraulik; tekanan retak

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*Corresponding author, email: zengfanhui023024@126.com